Thanks for responding.
I just typed in the answer, as I had done for the first two questions, which worked ok.
I gave a couple of incorrect answers to the first two questions, before getting the correct answers, but when I did give the incorrect answers the responses I got back were pretty clear prompts to try again, unlike the 'You're talking nonsense' response I got from the third question - it was as though the NPC had dropped the quest.
I've never found the dialogue option that starts "I know the answer to... (or whatever it says - some of the text in it looks a bit borked) to actually do anything other than tell me to type the answer anyway so stopped bothering to use it.
I'm just about to re-log so I'll try re-visiting to see if anything has changed.