Why do the think that the boiler stat is faulty

No good reason other than following the fault finding diagram has a
branch that suggests that you can have an operational (ie controls
temperature) thermostat but the pump does not overrun. The solution
they give for this is to replace the thermostat.

I take on board what you say though. And if fixing the wiring sorts it
out then I have the cheapest solution of all!

I've found a supplier of what I think is the replacement thermostat (it
seems to have been superceded by a newer model) and it's only =A318 so
not the end of the world anyway but thanks for the link. I'll file it
for future reference.=20

Mark