Postscript. The workshop manager had checked the caliper and found it was OK, though I think he said the outer pistons retracted together, or not together, or something. I didn't quite follow him. He suggested I avoid using the back brake while I rode home, but otherwise it'd probably be ok. So I did. I babied it, and the bike ran fine for about two miles, then the rear brake dragged again, then all but locked up again, exactly the same behaviour as what led to the whole situation a month and more ago, so I pulled over straight away. The caliper and disc were smoking hot, as I suspected. I was glad for my 8mm spanner – thanks, Igofar – and a nice little squirt of fluid out of the caliper freed it up temporarily. Somewhat remarkably I was able to ride the rest of the way home almost without touching even the front brake.
I know things will improve once I have wrapped my head around the braking circuit. I appreciate the knowledge in this forum, and I fear that it rather exceeds my vacillatory enthusiasm.