1. A method of signalling resource allocation data in a communication system which uses a plurality of sub-carriers arranged in a sequence of blocks, the method comprising: receiving an allocation of said sub-carriers for a user device, the allocation comprising data defining a starting block and data defining a number of consecutive blocks from the starting block; a first mapping of mapping the data defining the number of consecutive blocks from the starting block to a first mapped value in accordance with first mapping data; a second mapping of mapping the data defining the starting block and said first mapped value to a second mapped value representative of the allocation in accordance with second mapping data; and signalling said second mapped value as resource allocation data to the user device, wherein said first mapping data defines a mapping such that said first mapped value is less than or equal to the number of consecutive blocks allocated to the user device, and wherein said first mapping further defines a one-to-one mapping between said allocated number of consecutive blocks and said first mapped value.