Thanks Michael and Dave,

From some reading around and standing outside listening to it I've pretty much convinced myself that it's not a waterside or thermostat problem.

I have to admit that it's some time (ahem!) since it was serviced so dirt/soot on the photocell would be quite a possibility. It was installed about 17 years ago (doesn't time fly!) and I'm sensing that fuel starvation due to the oil pump wearing out is quite a popular fault on these boilers after a much shorter time than that. That might fit with the runs-for-a-bit-then-stops nature of the problem.

Having seen the dates on my old paperwork I think I owe it a service so I guess I'll get someone in to repair and service it at the same time - and brace myself for the inevitable list of other worn out bits to be taken into consideration!