Wotcha.
Well it's all finished now
There was an electrical issue which caused a bit of head scratching.
For some reason the bike was 'killing' solid state voltage regulators. You could run the dyanstart on a test rig and all would be fine - as soon as it went back on the bike there would be no output from the regulator.
To cut a long ( and expensive ) story short, I took the bike over to Peter at Classic Dynamo Regulator Conversions. He couldn't see any reason why this should be happening. Eventually, after much trying, testing, re-trying we finally found the culprit . . . . . . the battery. Once we tried another battery on it, everything worked fine.
I'd like to thank Peter Jury, of Classic Dynamo Regulator Conversion for his help and understanding in getting this problem sorted out. Neither of us had ever come across a battery causing this kind of problem. Still, it's all sorted now.
Took the bike for MOT - which it passed with no problems. Then it was off for a test run - which had to involve a cafe of course.