Welcome to the Worry Club!
In my (seriously) hectic last few days, I forgot to fill out part of the 98/2003. For the tapeworm/tick treatment, my vet signed and stamped and said "you can fill out the rest, can't you?" I said, SURE (I had done it for the rabies stuff), and then promptly forgot. Halfway through the flight, I remembered!
Got to the PETS office, and the first thing the lady said was - "there is a problem with your paperwork". I said, "I know - can't I just fill it out now?" She said, "no you cannot".
Fortunately, I had a receipt for the tick/tapeworm treatment and they released my cats to me. However, they still wanted a fax + original copy from my vet to confirm the date and time of the treatments.
Had I filled out the paperwork on my own, no one would be the wiser. I feel like an absolute fool for forgetting. The PETS lady very clearly told me how lucky I am that DEFRA was understanding with me, because mostly they are hardlined.