Thank you everybody for your input.
After behaving quite well this year, it disgraced itself at Banbury. I had to have a push start. After 15 miles it went down to one cylinder… and when I stopped to clean the plug it just would not go again. I’m fairly sure that that was magneto trouble in the heat. Of course, next day there was a great spark, but I’ll have to have the mag rebuilt again. My car experience tells me that when manetos are good, they are great; but when they are bad they are horrid! What appears to be a great spark outside the cylinder just won’t work under compression.
I think the problem may be also one of bore wear, maybe some castings which are slightly distorted over the years, & the cumulative effect leads to the problems.
I’ll keep you all posted, IF there is something positive to report!