Thanks for all the replies, turns out it was the fan!
Apparently cutting the gas off. Couple of squirts of WD40 sorted it.
Mother in law now has price for new fan, new boiler and hot water.
Thanks for all the replies.
I've heard that these boilers have a "high temp cut out" on the thermocouple.
Is this right and if so, I take it a standard, universal thermo won't do.
This would explain the outrageous quotes she's received.
Mother in law has an old Saunier Duval combi. Pilot light won't stay lit. When it's re lit and button released, pilot just gets smaller and goes out.
I know Corgi/Gas safe apply and I'm not after a do it yourself fix, just after pointer so she does'nt get ripped off.
Thanks