I've just turned my xbox on to find that my profile in PGR2 is 'damaged' I've seen people mention recalculating the checksums!?!? How do I do this and will it work!?!?

Well that's easier said than done actually. You need the signatures to sign the save game after you change the checksum. Unless you get TeamX4 to help you out it's quite unlikely you'll manage to do it.