
Thanks to all for advice. Fault is now traced and rectified.
Picture above shows the repair of the the pipe from thermostat to throttle box, using Loctite EA9492 high temperature epoxy adhesive, as recommended by Viral Jim. The adhesive is white.
I was perplexed that things improved after Marks DTM suggested I check for a spark at the plugs, while cranking engine by shorting solenoid lead to starter power lead. After removing the plug stick I turned the ignition key to crank; the engine cranked. After replacing the plug stick the engine started, ran briefly, then stopped. Removing plugstick got it going again, for as long as it felt like running, anything from 10 seconds to 90 minutes.
Today I tried unplugging the plugstick but not removing it. That got it going too, but not for long. With the ignition switched on, I tried wiggling the leads around the plugstick connector. As I did so, I heard clicks from the throttle box. I imagined a damaged lead in the harnesses. I wiggled each lead in turn, to locate a damaged lead. The guilty party was the cylinder head earth lead, which you see on the picture above to the left of the pipe repair. It was in place, but not quite tight. A quick tweak with a 10mm spanner, and all was well.
I keep starting it, now it's fine. Sounds unlikely, don't it?