BM, it's like the old joke: "How do I get to <wherever>?" "oooh, you don't want to start from here". Sorry to give an answer that's so much like that, but I wouldn't (personally) work without a written contract. I know it seems legalistic to be bothered about things like that, but it does make sure that both parties are working from the same assumptions.

Working on a project basis, I'd get the client to sign a functional spec - again, just because having these things written down can expose places where the client's assumptions are different to mine. Also, if it's not written down, then frankly I forget what I said. If not immediately, then certainly after a couple of months. Perhaps you could write some minutes of your conversation and get the client to agree (and sign) them?

andye's $0.02

Update: reread the original question and realised that I really wasn't answering it at all (sorry), and that you do have a requirements doc. All I can say is, perhaps you should take legal advice if you think it's worth it, otherwise maybe best to let it slide. Good luck in any case. andy.