Connector The present invention relates to a connector, methods for forming a connector and vehicles or enclosures which comprise a connector. More particularly, the invention relates to a connector for forming an enclosure of a vehicle. Goods vehicles for transporting goods by road often have an enclosure for containing goods to be transported. These enclosures may be part of a trailer for a lorry. However, they may also be formed as part of a vehicle such as a panel van. Figure 1 shows a lorry trailer according to the state of the art. The trailer comprises a generally rectangular enclosure 1. The enclosure has flat panel sidewalls 2, a flat panel front wall 3, and a flat roof panel (not visible). A connector rail 4 is provided between the sidewall 2 and the roof panel. A similar connector rail 5 is provided between the front panel 3 and the roof panel. A vertical post 6 is provided between the front wall 3 and the sidewall 2. Figure 2 shows a sectional view of the connector rail 4 shown in figure 1. The rail is used to connect the sidewall panel 2 of the trailer 1 to the roof panel of the trailer. The rail is manufactured from aluminium and has a blue painted (powder-coated) surface. The rail has a generally curved profile which is formed by the curved sheet of metal 6. A sidewall 2 of the enclosure fits into the space 7 and is retained by a fastener and an adhesive (not shown) which extends through a hole in the wall 8 which partially (along with wall 9) defines the space 7. The roof panel 5 sits on the flat surface 10 at the upper end of the connector and may be secured thereto. The roof panel is formed from a series of cross struts which have a ‘top hat’ cross section. These cross struts span the gap between the connector rail 4 and a corresponding parallel rail on the other side of the roof of the trailer. A skin is then located over the cross struts. It is amongst the objects of the invention to provide a vehicle or components for forming a vehicle or an enclosure which have a lower carbon footprint, are easier to manufacture, faster to manufacture, more cost effective to manufacture or less vulnerable to damage. In a first aspect the invention provides a connector for connecting two panels of a vehicle; comprising a first receiving portion and a second receiving portion, and a body which is located between the first and second receiving portions. Preferably each of the first and second receiving portions are adapted to receive separate panels of a vehicle. The receiving portions need not be adapted to receive panels directly. For example, the receiving portions may be adapted to receive or engage a strut or other member to which a panel may be connected. Preferably the body is curved and comprises two sidewalls which define a space therebetween and which extend between the first and second receiving portions. Preferably, both of the sidewalls are curved. Preferably, both of the sidewalls extend all the way between the first and second receiving portions. Preferably the connector is elongate. Preferably the direction of elongation of the connector is in a direction which is perpendicular to a plane defined by the curve of the body of the connector. The curve of the body may be a continuous curve or a discontinuous curve. The curve may be formed by a sidewall which has several straight sided portions. Alternatively, the curve may be formed by a sidewall which has a continuous gradient and/or a constant tangent. The body may comprise three or more sidewalls which extend between the first and second receiving portions. The curved body comprising the two sidewalls which define a space therebetween allows the connector to be made from a material which has a much lower carbon footprint than the metals (specifically aluminium) used in the prior art. They do so by providing structural rigidity which allows the use of materials such as materials which are plastic, comprise plastic or materials which are composites. This reduces the carbon footprint for manufacturing and operating the vehicle. The savings during manufacture can be significant due to the high energy input needed to manufacture aluminium. The weight saving by using plastic containing materials can also have a large impact on the carbon footprint to run the vehicles. The structure of the body of the connector also provides the benefit that if the connector is scraped by an object such as a bridge or a low-hanging tree branch, a hole in one layer may be less likely penetrate all the way into the enclosure defined by the connector (and any accompanying panels) because the second sidewall of the connector will remain intact. The enclosure can therefore remain water tight and preserve the goods which are contained in the enclosure. The connector may be made from a material which comprises plastic, or which is solely plastic. The connector may comprise or be formed from PVC (polyvinyl chloride). This feature has a synergy with the body being curved and comprising two sidewalls which define space therebetween. The plastic or the material comprising plastic has a lower carbon footprint than the metal layer and although the plastic material is structurally weaker than metal, the double sidewall feature allows the connector to have an acceptable rigidity. The connector has a lower carbon footprint because it is lighter than a corresponding metal connector. Over the lifetime of a petrol or diesel goods vehicle, the savings in CO ₂ from reduced fuel consumption are therefore significant. A brace member may be attached to the two sidewalls and bridge the space between the sidewalls. This provides enhanced rigidity to the connector and prevents twist of the connector, either during its manufacture, installation or use. The brace member may comprise a rib which separates two voids in the space between the two sidewalls. The presence of two voids separated by a rib provides a strong connector, yet one which is easily produced by extrusion. The orientation of the voids may be aligned with a length of the connector, wherein the length is defined as perpendicular to a plane which contains a curve of the body of the connector. More than one brace member may be attached to the two sidewalls and the brace members may bridge the space between the two sidewalls to form more than two voids in the space between the sidewalls. This provides an enhanced rigidity to the connector. The two sidewalls may converge towards the first receiving portion, such that a void closer to the first receiving portion has a lower volume than a void which is closer to the second receiving portion. The tapering of the volume of the voids allows the internal enclosure (which may be partially defined by the connector) to have the largest possible volume to accommodate goods, whilst maintaining a low carbon footprint, yet rigid, enclosure. The first receiving portion may comprise a flange to which a roof member may be attached. The second receiving portion may comprise a slot into which a sidewall of an enclosure may be received. The voids may be isolated from one another. This provides a connector which is less susceptible to damage because if one void is penetrated, the other voids will remain sealed off from any water ingress. The sealed nature of the voids also improves the rigidity of the connector. The brace member or brace members may give the connector a honeycomb internal structure. The voids may be completely enclosed. For example, the voids may have air-tight or water-tight barriers therebetween. The brace member or the brace members may extend over the length (and preferably over the whole length) of the connector, wherein the length of the connector is defined as perpendicular to a plane of the curve in the body of the connector. This improves the rigidity of the connector at the same time as providing a connector which is easily extruded from a material which is, or comprises, plastic. The first receiving portion may be provided with an engaging means for engaging a roof panel of a vehicle to the connector. The engaging means may be a fastening aperture. It may also be a rib or recess which is adapted to be adhered to a roof member such as a roof panel. The connector may be formed from an extruded length of material. This provides an easily manufactured connector which may be cut to length. It also provides an easily manufactured connector where the connector is made from plastic or comprises a plastic. One of the two sidewalls may be provided with an attachment means for connecting a light to the sidewall. The attachment means may be formed along the length, and preferably over the whole length, of the connector. The attachment means may be engageable with a feature on the light such that the light may be slid along the attachment means. This allows the light to be positioned adjustably, according to the goods in the enclosure. The attachment means may be a channel or ridge. The channel or ridge may be adapted to receive an engaging portion of a light. The attachment means may be located on the inside of the curve of the body of the connector, such that when the connector is connected to roof and sidewall panels of an enclosure via the first and second receiving portions, a light attached to the attachment means is located inside the enclosure and faces into the enclosure. The connector may be formed from a plastic and the plastic may be impregnated with a pigment which has a different colour from the plastic itself. This further reduces the carbon footprint of the vehicle or enclosure because the connector does not need to be painted. Painting, such as powder-coating, is a high carbon emission process. This is due to the usual transportation of components necessary to a specialist painting facility, and the nature of the painting process itself). The appropriate colour pigment can instead be incorporated into the plastic during the manufacture. This also improves the scratch- resistance of the connector. If a painted connector is scratched, the scratch might penetrate the paint layer, leaving an obvious scratch. In the invention, because any colour pigment will extend all the way through the layer which comprises the plastic, scratches will not penetrate to an underlying metal layer of a different colour and will therefore be much less visible. Preferably, the cross section of the connector is constant over its length. This provides an easily extruded connector. In a second aspect the invention provides a vehicle having an enclosure which comprises a vertical wall panel and a roof panel which are connected by a connector as described herein, wherein the roof panel is received in the first receiving portion of the connector and the vertical wall panel is received in the second receiving portion of the connector. The enclosure may comprise two, three or four connectors according to the present invention. These connectors may connect different edges of a roof panel to corresponding edges of sidewalls, front walls or back walls of the enclosure. The enclosure may also comprise generally vertical pillars which connect two generally vertical walls of the enclosure. The generally vertical pillars may be formed of plastic or a material which comprises plastic. The vertical pillars may comprise a metal structural component and a non-structural component which comprises (or is) plastic. The non- structural component may be a capping which forms an external surface of the enclosure. The plastic capping may cover the structural component. This further reduces the carbon footprint of manufacturing and operating the vehicle. The vehicle may be a lorry, a van or a trailer which is adapted to be towed by a vehicle. In a further aspect of the invention, there is provided a method for forming a connector as described herein, comprising the step of extruding a material which comprises plastic to form the connector. In a further aspect of the invention there is provided an enclosure which comprises a connector as described herein and one or more walls and a roof member which are received in the receiving portions of the connector. An embodiment of the invention will now be described with reference to the figures of the drawings, in which; Figure 1 shows a vehicle trailer according to the prior art. Figure 2 shows a connector according to the prior art. Figure 3 shows a cross-sectional view of a connector according to the invention. Figure 4 shows a cross-sectional view of a connector according to the invention when engaged with a roof panel of a vehicle. Figure 5 shows a top perspective view of roof panel of a vehicle which is attached to three connectors according to the present invention. Figure 6 shows a top perspective view of an enlarged corner portion of figure 5. Figure 7 shows a top perspective view of an enlarged corner portion of a vehicle enclosure according to the invention. Figure 8 shows a plan cross-sectional view of the capping 53 shown in the figure 7. Figure 9 shows a side cross-sectional view of a base of a vehicle according to the invention attached to the sidewalls of the enclosure of the vehicle. Figures 1 and 2 are discussed in the introductory section above. Figure 3 shows a cross-sectional view of a connector according to the invention. The connector 30 is attached to a vertical sidewall 32 of an enclosure of a vehicle. The vertical sidewall 32 is received in a slot 36 which is formed by two sidewalls 33 and 34. The sidewall 34, which contacts an outer surface of the vertical sidewall 32, extends downwardly from the upper end 35 of the slot 36. It extends downwardly further than the opposing sidewall 33. The sidewall 34 is generally straight, but is provided with ribs 37, which form protrusions when viewed in a side-section as in figure 3. The ribs run all the way along the length of the slot 36. The length of the slot is defined as being perpendicular to the plane of figure 3, which is perpendicular to the plane which defines the curve of the body (discussed below) of the connector. The function of the ribs 37 is to grip the sidewall 32 when it is inserted into the slot. They also assist in retaining any adhesive in the slot whilst it sets, to retain the sidewall 32 in the slot. The ribs may be pointed or flat-topped. The sidewall 33 of the slot 36 extends downwardly from the upper end of the slot. The sidewall 33 is slightly curved and resiliently deformable such that when the sidewall 32 is inserted into the slot 36 the sidewall 33 grips the sidewall 32. At its lower end, the sidewall 33 is provided with a rib 38 which extends towards the opposing side wall 37. The rib has a pointed cross section. The rib 38 helps the sidewall 33 to grip the sidewall 32 when it is inserted into the slot 36. It also helps to retain any adhesive present in the slots whilst the adhesive sets to hold the sidewall 32 in place. The body 41 of the connector extends upwardly from the rib 39. The body of the connector comprises an outer curved sidewall 40 and an inner curved sidewall 42 which are spaced apart. The radius of the curvature of the outer sidewall 40 is greater than the radius of the curvature of the inner sidewall 42 such that the outer sidewall and the inner sidewall converge with one another. That is, the inner sidewall and the outer sidewall of the body become closer to one another the further they extend from the slot 36. The inner and outer sidewalls are curved such that their upper portions lie generally perpendicular to the vertical sidewall 32 and the sidewalls 33 and 34 of the slot 36 which receives the sidewall 32. The space between the inner and outer sidewalls 40 and 42 is separated by ribs 43 such that the space is divided into five separate compartments 44. Each rib 43 extends all of the way between the inner and outer sidewalls. Due to the convergence of the inner and outer sidewalls the length of the ribs decreases the further the ribs are from the slot 36. The ribs 43 are straight sided and provide rigidity to the connector by preventing the inner and outer sidewalls from twisting relative to one another. Each of the compartments 44 are completely sealed from each of the other compartments 44. However, in some embodiments there is provided an aperture in one or more of the compartments so that a user can access the internal space of the compartments. This can be useful to insert, for example, insulation into one or more of the compartments. The compartments can provide a useful place to house installation when the enclosure, of which the connector forms a part, needs to be insulated to preserve the goods contained therein. The upper end of the connector is provided with a horizontal flange 45. The roof panel 46 of the enclosure sits on the horizontal flange 45 and is attached thereto, either by an adhesive or by a fastener such as a bolt. Although figure 3 only shows a cross sectional view of the connector the cross section of the connector is constant over its length. The length of the connector is defined as perpendicular to the plane which is shown in figure 3, i.e. perpendicular to the plane which defines the curve of the body of the connector. A plastic capping 47 is shown. This is attached over the outside sidewall 40 of the connector. The plastic capping 47 fits into a bent portion 48 at an upper end of the body of the connector, where it is retained. The plastic capping of the connector has an upper end 49 which is bent to conform with the bent portion 48 of the body of the connector. The body of the connector is made from polyvinyl chloride (PVC). The PVC is impregnated with a blue pigment. This pigment is distributed throughout the material which forms the inner and outer sidewalls 40 and 42, the sidewalls 34 and 33, the flange 45, the bent portion 48 and the ribs 43. The plastic capping 47 is also made from PVC and has the same pigment distributed throughout its plastic structure. Figure 4 shows a view of the connector 30 shown in figure 3, attached to the roof panel 46. The roof panel is provided with a strut 50 which spans the distance between opposite sidewalls 32 (not shown in figure 4) to provide strength and rigidity to the roof panel. In the embodiment shown in figure 4 the roof strut 50 is parallel to the connector 30. In other embodiments the roof strut 50 may be perpendicular to the length of the connector 30 such that the brace strut 50 is received by the flange 45 of the connector. Figure 5 shows an assembly comprising a roof panel 46 and three connectors 30, 30’ and 30’’. The connectors have the general structure shown in figure 3. A first connector 30’ runs along one side of the roof panel 46 and receives the roof struts 50 on its flange 45. The second connector 30’’ runs parallel to the first connector along the opposite side of the roof panel 46. A third connector 30 runs along a front edge of the roof panel 46 and is connected at either end to the first and second connectors 30’ and 30’’. Each of the connectors may be connected to a corresponding side or front wall which fits into the slots 36 on each connector to form three sides of the enclosure. In this embodiment the assembly is a roof portion of a trailer which is detachable from a lorry. Figure 6 shows an enlarged portion of a front corner of the roof assembly shown in figure 5. The roof struts 50 are shown sitting on the flange 45 of the connector 30’. The connector 30 along the front edge of the roof assembly is connected to the first connector 30’ by a corner capping member 51. The capping member is riveted 52 to each of the connectors and covers the gap between the connectors. The two connectors are held at right angles to one another. Each of the connectors shown in figures 5 and 6 is provided with its own plastic capping portion 47 as in figure 3. Figure 7 shows part of an enclosure which includes a similar arrangement to figure 6 but from a corner perspective view. A vertical plastic capping 53 extends downwardly from the capping member 51 which caps the corner between the connectors 30’ and 30. A front panel 54 of the enclosure is shown extending downwardly from the connector 30. A side panel 55 of the enclosure is also shown. This side panel is received in the slot 36 of the connector 30’. Figure 8 shows a plan cross sectional view of the vertical plastic capping 53 and associated components which are shown in figure 7. The plastic capping 53 forms an outer surface of the enclosure. The inner surface of the front panel 54 and the side panel 55 are joined together by a brace 56. The brace is made from metal and provides the structural connection between the front and side panels 54 and 55 of the enclosure. The plastic capping 53 is not structural. The brace 56 is connected to the side and front panels 54 and 55 by a fastening means or by an adhesive. The space 64 between the vertical plastic capping 53 and the brace 56 may be filled with insulation should the contents of the enclosure require protection from thermal changes. Figure 9 shows how the side panel 55 shown in figure 7 is connected to the base 57 of a lorry trailer which comprises the enclosure. Connector 58 is provided between the side panel 55 and the base 57. The connector 58 is provided with flanges 59 which engage the base 57 and a slot 60 which engages the sidewall 55. The slot 60 is formed by opposing walls 61 and 62. These walls are provided with ribs or recesses 63. These ribs or recesses help the walls 61 and 62 to grip the side panel 55. They also help to contain any adhesive whilst it is setting. Connectors according to the invention may be manufactured by an extrusion process. This involves forcing heated material which comprises a plastic component or is wholly plastic through a die which generates the structure of the connector body, the sidewalls 34 and 33, the flange 45 and the bent portion 48. The advantage of using an extrusion process to form the connector is that connectors of any desired length may be made. Enclosures of custom sizes may therefore be made cost effectively. The skilled person is aware of several types of extrusion process which may be used to manufacture connectors as described herein. Example 1 The following tables show hypothetical examples of the invention and the reductions in carbon footprints which are provided in both the manufacture and the operation of vehicles which incorporate enclosures according to the invention. Table 1 shows the dimensions and weight of four components of an enclosure according to the invention. The plastic capping component corresponds to the plastic capping 47 shown in figure 3. The ‘roof rail side’ corresponds to the connectors 30’ and 30’’ shown in figure 5. The ‘front pillar’ corresponds to the vertical plastic capping 53 shown in figure 7. The length of each of these components is shown. The total amount of CO ₂ which is emitted by the production of the components is estimated in table 1 and their weight is also shown. Table 2 shows the corresponding data for the components shown in Table 1 when they are manufactured from aluminium and according to the prior art design shown in figure 2. Table 2 also shows the surface area of the aluminium components which requires painting. Table 1: Table 2: Table 3 shows a comparison of the total weights stated in tables 1 and 2 and the weight saved per vehicle. TheCO ₂ saved during the manufacture of each vehicle is also estimated. The ‘CO ₂ saved per annum’ figure is calculated based on the building of 40 vehicles per week. Table 3: The reduced weight of examples according to the invention can also have a significant impact on the CO ₂ emissions over the lifetime of a vehicle. Although data is sparse in connection with trucks/trailers, it is thought that for a 100kg weight reduction in a medium sized car, the CO ₂ reduction is around 7.6 g/km. In the present example, with a weight saving of 40kg per vehicle the CO ₂ saving could be around 3.04 g/km. Over the vehicle’s lifetime (or a fleet of vehicles) in which high hundreds of thousands of miles plus can be covered by each vehicle, the CO ₂ savings are substantial. The amounts of CO ₂ which are recited in tables 1 and 2 are based on the following assumptions for producing one kilogramme of either PVC or virgin aluminium and the painting of one metre squared of surface area.