Hey Grumpy,
You strike me as a man of both extensive knowledge and experience (each no doubt has been a result of the other). I expect you have also experienced the phenomenen as I have, of fixing things by just "having a look at them"!!!!
You pull them apart to find a fault, cant find anything, put it back together and away it goes!!!!
After your last helpful post I checked the magneto gap with a card, tested the resistance over the primary and HT wires and put it all back together. BINGO I now have spark. I did not alter anything, just looked at it!!!
Is this a variation on "Sods Law"??
Sadly I think it means there is a cracked or broken wire which I have disturbed to make connection again and in due course it will let me down again.
Will put it all back together tomorrow and report back. In the meantime your comments on my experience would be greatly valued.
Jim