Anyway, I stopped five miles from home to take a souvenir photo, and just could not get the thing to start again. Then I realised that the battery was flat so I phoned my sister-in-law to collect me and the battery so I could recharge it before going back to the bike again.
While I was waiting, through the wonders of technology, I put a photo and message on the RealClassic Facebook page, and immediately got some helpful comments. First of all, a bike with a magdyno doesn't need the battery to start it - who'd have known! Some people have had vapour locks in the carb making hot starting very difficult, and the problem also sounded like it could be the capacitor. Hmmm. Or maybe I just hadn't worked out the right hot starting technique for this particular machine.
Incidentally, as I was waiting by the bike, three separate Harley riders stopped and asked if I was OK, and then a suave young man in a red Ferrari stopped too. I assumed that MadMike had come to my rescue as his number plate was V8 MAD. He asked if there was anything he could do and I suggested he swap vehicles with me, but he just laughed for some reason...
Two hours later, battery back on the bike (but still flat) and she started fine after a couple of kicks (with the engine now back to normal temperature). Got home with no problems and with that warm satisfied feeling you have after a good ride.
Now there's some investigating to do before trying out longer distances, but this is why I enjoy these old clunkers.