I first started this posting with:
Before I go completely mad, I better consult the collective wisdom here...
But then I went back and solved the problem, purely by chance. Hurray.
As a warning and education, I'll post about the problem anyway.
I have a 1976 LHD Triumph Spitfire 1500, with the old Herald/TR6 switchgear on the steering column (pre-FH100020). It's in good nick, complete and most of the time it's in perfect working order. But while driving it today, I noticed the indicators had stopped working. On closer inspection, the stop lamps, reverse lamps, tachometer, fuel and temp indicators and heater fan switch had also stopped working.
"Ah", I thought, "that'll be a fuse, then". Sure enough, according to the electrical diagram, all of these things are fed through the same fuse, the top one in a set of three. But there was nothing wrong with the fuse.
So I got out the multimeter and started doing some measurements. There was power going to the fuse (three white wires, all live) and there was power coming from it, through four green wires. But at the other ends of all these green wires, there was no power!
Bother. :-(
I really thought I was going mad, I had already started writing to the therapeutists here, when I decided to check everything one more time. My lovely wife, whose knowledge about electrickery on a scale of 1 to 10 is about -5, was watching what I was doing. As I was applying the multimeter to the wires at the back of the fuse box, she said, while looking at the dashboard, "there's something working here... oh, it's gone out now.... no, it's working again". It was the seat belt warning light, which is also fed by this fuse.
So the problem was in the fuse box. I used some tuner spray on it and everything is working fine again. God knows for how long, though. Of course it's only now that I remember a similar problem being discussed in this very newsgroup, not so long ago: