I would agree with all that you said Mike except......
My M75 developed a similar fault in that it would run for about 20 minutes and then lose power and stop. Using the booster, which is independantly controlled, I could extend the running a little but this confirmed it was ignition.
After 30 mins or so, it would start and run ok for another 20 minutes.
I had new coils fitted in the mags and it has been fine eversince. Tests confirmed the coils failed a hot test.
Seems a huge coincidence but they were both of a similar vintage and broke down over time in the same way.