I think I have got to the bottom of it
Since my last post I have fitted a new coil and ridden quite a bit with no problems. Went to a vintage/classic bike rally today.....great day. Started the bike to leave and it immediately cut out and would not fire up again. Note that bike has had a new pick up coil and new ignition coil. So.....off with the seat, check igniter plug. I moved it while cranking and she fires up! While she is running I was able to move the plug and she stops. Some evidence of the seat pan touching the plug, probably when hitting bumps in the road. So I ride home with no incident. Next step is to thoroughly check out the plug and grease it with dielectric grease. As has been said 50% of "coil problems" are something else. I feel really stupid....sometimes it is so easy to overlook the obvious. I will make it a rule.......if I think the problem is electrical, check connections first. Interesting to note that the problem manifested when ambient temperature had risen to 25C or so. I guess that a poor connection gets worse as temperature rises........resistance increases in proportion to temperature? Expensive lesson but at least I have a spare pick up coil and ignition coil to carry with me.