You need to phone the helpline. You certainly should have given them the 06-07 figure last year. Give them the 07-08 figure and confirm that by doing so (& giving any other information needed) you are doing the renewal on the phone. Deal with that first so that the 07-08 renewal is done.
Then ask about the second review form they have sent, and tell them you gave them the 06-07 figure last year. If they say they haven't got it there is something very wrong with their records (if you didn't give it to them last year they should have stopped your tax credits last autumn). However, do the best you can before ringing them to have a good estimate of your 06-07 income just in case - but if you do have to give it make it clear it is an estimate.
Get the name of the helpline person you speak to & keep a note of it, with the date and time of the call, in case there are any queries later on.