Turns out it was the points. Bosch points, only 7000 miles on them, but the follower was worn, and the contacts very pitted. Despite having that ballast resistor. Really surprising they deteriorated so quickly.

Replaced the points, set the gap to 0.017", started fine. Dwell was 50 so I left the points at 0.017.

Engine starts immediately, idles smoothly at 800 rpm, and purrs like a kitten at speed.

I'd never have thought it was a points problem - but you live and learn!
Tom