sheldrake

If someone does that my advice would be to respond with nonsense.
"Of course you can lend me 200 pounds but I don't need the money right now but thanks anyway for the offer I may take you up on it next week perhaps" or "Yes indeed my favorite tea would be Earl Grey except on a Tuesday". It is very important you don't forget to give them a friendly nod and put a face on like you just answered their question.
10 to 1 they start doubting their selves and will forget the original question. Just remember to never repeat the answer. If they ask you what you mean act a bit annoyed and tell them it shouldn't be that difficult to understand even for them and you not going to explain the whole thing to them twice. Or excuse yourself and pretend you misheard the question to give you time to think about a proper answer.
The basis theory is to do what has been done to you. If someone surprises just surprise them back. This is also applicable for annoying. The best thing to do if someone annoys you is to annoy them but better and more professional. I do a lot of work over the phone and if you ever get insulted or obscene phone calls just pretend the line is bad and you can't here them. Hello. . . . . hello. . .? and tap the phone a couple of time on the table which makes a horrific noise on the other side of the phone. Tonde