Electricians came back this afternoon, took up most of the upstairs floorboards, and traced the problem to a screw in a junction box which hadn't been tightened. So they tightened it, and now everything works again.

While they were there, though, they also noticed that my ring main fails to be a ring; so they downgraded me from a 32A to a 20A fuse on that circuit on general principles of safety, and they recommend I have it all redone properly at a later date.

Now I have to go out shopping: rapid power-cycling appears to have comprehensively nadgered my radio-synchronised alarm clock, with the result that it (a) can't radio-synchronise any more, and (b) won't make alarm noises. It does still work as a clock, if you manually set it, but that really isn't enough to be useful.