I think it's fixed.

Bought a new PCB, (list price inc Vat about £200), on e-bay for £16
plus postage. The logic being that at that price it could be dumped
with the boiler if it didn't work.

New PCB (and it was brand new and boxed), gave same symptoms, i.e.
shutting down too soon. I assumed that the problem must be in the wet
side of the system, a blockage preventing good flow perhps. So I
drained and flushed it.. Also stripped the pump which looked OK. Same
symptoms.

The thermistors had been checked originally and were OK. Swapping them
(they are all the same) made no difference.

Had a good look at the circuit diagram and saw that whoever had worked
on the boiler last had connected the flow thermistor to the dry fire
sensor wire and vice versa. This meant that the PCB was getting a high
reading on what it thought was the flow and then shutting down too
soon. The PCB was doing it right! Connecting the wires as per the
diagram fixed it.

Now, who did it? I know I didn't but I did have a new burner fitted by
BG in the summer, since when the boiler has not been much in use, (warm
weather and our being away). The wiring would have had to be
disconnected from the thermistors to do it. Perhaps it was BG or maybe
the tooth fairy.

At at least I have only wasted £16, plus postage of course!

Naffer