In message , T i m
writes

Strangely though, we rang a relative of theirs who had just been
through similar, he had bought a new control unit and found it no
better. Called a heating engineer in who diagnosed a low gas pressure
setting (not sure what / where), suggesting that it had never been
installed properly in the first place but had been working fine for
the last 3 years?


Actually - I forgot about that possibility, although it's unlikely

pilot comes on - pcb senses the flame, opens the main gas valve, local
pressure drops, pilot goes out, no flame sense


--
geoff