A ROOFING BRACKET FIELD OF THE INVENTION The present invention relates generally to roofing and more particularly to a bracket for attaching a metal roof sheet to an underlying purlin of a roof 5 structure. BACKGROUND TO THE INVENTION Roof sheets made of metal are commonly used in the roof structure of buildings. In most instances, the roofing sheets are secured to the horizontal support beams of the roof structure, otherwise known as purlins, with a series of 10 spaced apart roof clips which lock into ridges in the roof sheet. The spacing between the roof clips is determined by the size and shape of the roof sheet's profile. In order to make the process of securing roof sheets to purlins quicker, the roof clips are commonly purchased off the shelf mounted to a metal rail at the correct spacing to suit the particular roof sheets being used. This arrangement is 15 known as a rail of clips. The rail typically has three roof clips and is fastened with screws to the purlin prior to securing the roof sheets thereon. As metal is a good conductor of heat, a building with a metal roof is inefficient and requires some form of insulation, for example an insulation blanket made of glass wool or the like, such that the temperature within the building can 20 be efficiently controlled. In this respect, a layer of insulating material is typically positioned on top of the purlins, with the rail of clips and roof sheets being laid on top of the Insulating material. A problem with insulating a roof in this manner is that areas of the insulating material which directly overlie the purlins are compressed between the purlins and the rail of clips securing the roof sheets, as 25 shown in Figure 1. As a result, the compressed areas of insulating material have a reduced resistance to heat flow which causes the overall thermal efficiency of the insulating material to be lower. In the past, attempts have been made to overcome the above mentioned problem by increasing the thickness of the insulating material. However, this can 30 result in the roofing sheets being difficult to properly secure to the rail of clips as the bulk of the insulating material pushes the roof sheets away from the clips. In addition, the bulk of the insulation makes it more difficult to screw the rail of clips to the purlin. This data. for aonlication number 2009236041. is current as of 2014-05-23 22:13 AEST 2 Attempts have also been made to address the above problem by locating elongated blocks of polystyrene on top of the insulating material where the rail of clips are to be positioned. Although the elongated blocks compress the Insulating material, the thermal efficiency of the polystyrene to some degree compensates 5 for the compression. However, as polystyrene is relatively lightweight the elongate blocks are readily blown around the installation site and also out of their correct position on top of the insulating material prior to being fastened in place. In addition they increase the overall installation time of the roof. Other attempts have been made to address the above problem by 10 providing an insulation spacer between the purlin and the rail of clips. The insulation spacer includes an upper support surface upon which the rail of clips can be positioned, and a series of supporting columns which rest upon the insulating material and purlin, as shown In Figure 2. A problem with the insulation spacer shown in Figure 2 is that the spacer 15 adds to the cost and installation time of the roof. In this regard, the supporting columns of the insulation spacer firstly need to be fastened to the purlin. Then, the rail of clips need to be fastened to the upper support surface of the insulation spacer. It would be desirable to overcome or alleviate at least one of the 20 abovementioned problems with the prior art. Any discussion of documents, devices, acts or knowledge In this specification is Included to explain the context of the invention. It should not be taken as an admission that any of the material formed part of the prior art base or the common general knowledge in the relevant art in Australia or any other 25 country on or before the priority date of the claims herein. SUMMARY OF THE INVENTION In accordance with the present invention there Is provided a bracket for attaching a roof sheet to a purlin of a roof structure. The bracket includes a platform for supporting the roof sheet thereon, a plurality of support legs 30 extending from the platform, and a plurality of roof clips positioned on the platform for securing the roof sheet to the bracket, wherein the support legs are fastenable to the purlin such that a compressible insulation material positioned between the This data. for application number 2009236041. is current as of 2014-05-23 22:13 AEST 3 bracket and the purlin is prevented from being compressed by the platform in regions between the legs. The platform of the bracket is preferably of a unitary construction and the roof clips are preferably each individually mounted directly to the platform. In 5 addition, the support legs are preferably mounted directly to the platform. The present invention advantageously provides a bracket which can attach a roof sheet to a purlin of a roof structure whilst at the same time minimise the regions of insulation material which are compressed between the bracket and the purlin of the roof structure. Accordingly, roof sheets can be rapidly and efficiently 10 installed, thereby reducing labour costs. In addition, the insulation material is advantageously able to maintain an overall higher thermal efficiency (R-valve) than insulation material installed in accordance with the prior art. BRIEF DESCRIPTION OF THE DRAWINGS A preferred embodiment of the invention will now be described. The 15 preferred embodiment should not be considered as limiting any of the statements In the previous sections. The preferred embodiment will be described with reference to the following figures. Figure 1 is a photograph of a rail of clips being installed directly to a purlin of a roof structure, in accordance with the prior art: 20 Figure 2 is a photograph of a rail of clips and an underlying insulation spacer being installed on a purlin of a roof structure, in accordance with the prior art; Figure 3 is a photograph of one end of a bracket for attaching a roof sheet to an underlying purlin of a roof structure, according to an embodiment of the 25 invention; Figure 4 is a side perspective view of a bracket for attaching a roof sheet to an underlying purlin of a roof structure, according to another embodiment of the invention; Figure 5 is a side view of the bracket illustrated in Figure 4; 30 Figure 6 is a top view of the bracket illustrated in Figure 4; and Figure 7 Is an end view of the bracket illustrated in Figure 4. This data, for application number 2009236041, is current as of 2014-05-23 22:13 AEST 4 DESCRIPTION OF PREFERRED EMBODIMENT With reference to Figure 1 of the accompanying drawings there is shown a rail of clips 1 in accordance with the prior art with one end of the rail 1 secured to a purlin 3 of a roof structure. A layer of insulating material 5 is positioned on top 5 of the purlin 3 with the insulating material 5 being compressed where the rail of clips I overlies the purlin 3, thereby reducing the thermal efficiency of the insulating material 5 and subsequently the overall roof structure. With reference to Figure 2 of the accompanying drawings there is shown an insulating spacer 7 between the rail of clips 1 and the purlin 3 with support 10 columns 9 of the insulating spacer 7 positioned on top of the insulating material 5, in accordance with the prior art. The spacer 7 includes a support deck 11 which Is supported by three spaced apart support columns 9. In order to secure the rail of clips I to the purlin 3 the insulating spacer 7 is firstly positioned on top of the insulating material 5, The support deck 11 has three apertures which overlie the 15 support columns 9. By Inserting a fastener through each aperture and into a bottom face of each support column 9, the spacer 7 can be subsequently fastened to the purlin 3 through the insulating material 5. With the insulating spacer 7 securely in place, the rail of clips I can then be fastened with screws or the like on top of the support deck 11 of the insulating spacer 7. The installation 20 of one rail of clips 1 and one spacer 7 requires a total of six screws. With reference to Figure 3 to 7 of the accompanying drawings, there is shown a bracket 13 for attaching a roof sheet to a purlin of a roof structure, in accordance with the present Invention. The bracket 13 includes a platform 15 for supporting a roof sheet thereon, a plurality of support legs 17 extending from an 25 underside of the platform, and a plurality of roof clips 19 positioned on the platform 15 for securing the roof sheet to the bracket 13. Preferably, a total of three support legs 17 are provided with each support leg 17 having a length which approximately corresponds to the thickness of the insulation material being installed. The support legs 17 are fastenable to a purlin such that the Insulation 30 material positioned between the bracket 13 and the purlin is prevented from being substantially compressed by the platform 15 in the regions 21 underneath the platform 15 between the support legs 17. The roof clips 19 are fastened directly to the platform 15 during manufacture by fasteners, for example screws or bolts This data, for application number 2009236041. is current as of 2014-05-23 22:13 AEST 5 23 as shown in Figure 3. Alternatively, as shown in Figures 4 to 7, the roof clips 19 may be fastened to the platform 15 by some other means, for example welding, riveting or an adhesive, during manufacture of the bracket 13. The roof clips 19 are conventional clips used to secure roof sheets. The spacing between 5 adjacent roof clips 19 and the shape of the roof clips 19 is dependent upon the profile of the roof sheets being installed. The platform 15 is preferably of unitary construction and has downwardly projecting sidewalls 25 on opposite sides of the platform 15 to provide the platform 15 with a generally U-shaped profile. One end of the platform 15 includes a protruding tongue 27 which Is adapted to 10 interengage with an opposite end of the platform 15 of an adjacent bracket 13. As shown in Figure 3, the support legs 17 of the bracket 13 may be fixed to the platform 15 by welding, riveting or the like. In order to install the bracket 13 a layer of insulating material 5 is firstly positioned on top of the purlin 3. The bracket 13 is then positioned on top of the 15 insulating material 5. The support legs 17 compress the insulating material 5 and a fastener is inserted through a support leg 17 and into the purlin 3. In this respect a total of three fasteners is required to secure the bracket 13, one fastener being associated with each support leg 17. Once the bracket 13 is securely in place the roofing sheet can be attached to the roof clips 19 of the 20 bracket 13 in a conventional manner. The protruding tongue 27 of another bracket 13 can then be inserted into the opposed end of the first bracket 13 to thereby link the brackets 13 together such that roofing sheets can continue to be mounted. The bracket 13 of the present Invention advantageously enables roof 25 sheets and insulating material to be installed to the purlins of a roof structure in approximately half the time compared to the Insulating spacer 7 and rail of clips 1 combination of the prior art. In this respect, as the roof clips 19 are fixed to the platform 15 during manufacture the roof installer is only required to fasten the support legs 17 of the bracket 13 to the purlin. In this regard, the bracket 13 30 advantageously requires only three fasteners to fix the bracket 13 in position over the purin. In comparison, the insulating spacer 7 and rail of clips 1 combination of the prior art required three fasteners to attach the rail of clips I to the insulating spacer 7 and a further three fasteners to attach the insulating spacer 7 to the This data. for anolication number 2009236041. is current as of 2014-05-23 22:13 AEST 6 purlin. Further, the bracket 13 of the present invention advantageously utilises approximately 30-40% less material than the combination of the rail of clips I and insulation spacer 7 of the prior art. As the present invention may be embodied in several forms without 4 5 departing from the essential characteristics of the invention, it should be understood that the above-described embodiment should not be considered to limit the present Invention but rather should be construed broadly. Various > qualifications and equivalent arrangements are Intended to be included within the spirit and scope of the invention. 10 This data. for application number 2009236041. is current as of 2014-05-23 22:13 AEST