A reader pointed out a set of hidden spam links that had attached themselves to the footer of the site.
I’d noticed them previously and thought that I’d corrected them (see comment). But they turned up again. Turns out it was from an interesting flaw in the website, and was being inserted from outside. I’ve hardened up the problem area(s) and I’ll monitor for a repitition.
It is likely that I’ll need to take the site down for a few hours in the early morning on Sunday to do some further maintenance. So don’t get too surprised to get a unexpected screen around that time.