OK, so I came back to the real world and started tidying, and what should I realize when I finally tackled the mounds of paper on my admin desk? That the fee which had been dangled in front of me as an enticement to do a day's workshops, workshops now long enough in the past for me to have forgotten all about the matter, had never been paid.
When I think about it now, I don't think those who invited me ever had my address to send a cheque to, since everything's done through emails now. But why had no one asked for it or asked me to submit an invoice?
It's probably just an oversight. Well, I hope so, and that it's not another instance of the depressingly widespread assumption that writers should just be grateful for any work that comes their way and that money's beside the point.