Many thanks for your replies. As luck would have it, I have a capacitor tester. (I'm into old radios) So I can have a look at that. I shall take the pump off in the morning as suggested, and see what turns !
I would like to invest in a combustion tester. If you have any thoughts on something...
A couple of months ago my boiler, a Thermecon 40/60 developed a noise that sounded the same as one I had first heard a few years ago. That resulted in the oil pump seizing up due to an oil leak. The oil had washed away the bearing grease. The pump was renewed, and all was well.
This time it...