1. A method of identifying an update between a first version of a data file and a second version of the data file, the data file having a plurality of blocks of data, the meaning of the data file being insensitive to an ordering of the blocks of data within the data file, the method comprising the steps of: providing each of said plurality of blocks of data with a first checksum; providing each of said versions of the data file with a second checksum of the version of the data file as a whole, said second checksum being insensitive to the ordering of the blocks of data within the data file; comparing the second checksum of the first version of the data file with the second checksum of the second version of the data file; responsive to said comparison indicating that the second checksum of the first version of the data file differs from the second checksum of the second version of the data file, comparing the first checksum of each of said plurality of blocks of data of the first version of the data file with the corresponding first checksum of each of said plurality of blocks of data of the second version of the data file; and providing an indication of which of said plurality of blocks of data differ between the first version of the data file and the second version of the data file.