TOPIC:

I found an almost broken wire on the positive feed to the coils so I fixed that (it was one of those horrible blue connectors which cut the insulation) and fitted a new ignition unit. In 5 or 6 20 mile trips (I live on a small island) it has only cut out once so I'm hoping that was a one off. I've connected a light bulb to the coil feed so if that stays on when it next cuts out I'll know the switches are OK and that the problem must be the 49 year old coils.
Process of elimination.
John.

After checking all these helpful suggestions, and finding nothing else wrong, I finally fitted 2 new coils and hey presto, bike now runs like a charm.
I tested the old coils (49 years old, after all) and found infinite resistance on both secondary coils. I'm surprised it ran at all. I guess the fine wire in the secondary coils breaks down after this length of time but that the high voltage is still enough to generate a spark until the wire gets hot.