Hi
Thanks for the input .I will check the battery with a load test but it does live on an optimate and that says its ok.The coil connectors are fine and as both the cylinders die it's possibly only somethig that is common to both like the cdi unit which I hope to replace with a used item soon or the pulse generator.I have yet to inspect this but think it would be unlikly to run fine then suddenly cut out only to start and run fine again.
I hope it is the cdi unit just breaking down after a few minutes use under load.If replacing it does sort it out I'll post the findings and we all might learn somthing as I have not had this problem in about 25 years of tinkering with bikes.
Thanks again
Alec