I have a problem with my central heating boiler, a Baxi WM 38/3 RS. After
the burners have been lit for 20 minutes they turn off and the pilot light
is out. I can immediately re-light the pilot but if I start the boiler
again, the pilot goes out and the burners do not start. If I light the pilot
and leave it for 10-15 minutes, the burners will fire up again but only for
the 20 minute period. Looking at the fault guide it say that a cause of
nuisance shut downs is a dirty or loose thermocouple connection; defective
thermocouple or a defective power unit. I'm unwilling to do any diy on a gas
appliance so have spoken to a plumber (BUT I have checked that the
thermocouple is not loose). Over the 'phone he has suggested the boiler is
near its end of life (its 12 years old) and that the fault is probably with
the gas value which will cost £150+ and labour cost to replace. Is this
reasonable in the opinion of the members of this group?

Thanks for any contributions.