In article ,
"Stoney" writes:
This my third go at getting somewhere with my ailing boiler.
The problem now appears to be that the thermocouple is failing at high
temperatures. So it will stay on all night but it will stop working after
the boiler runs for 10 minuites or so.
Does anyone have any experience of this?
I have put in a new thermocouple but the same thing is happening.

Yes, I've seen the same problem with a gas fire which Eastern Gas fitted.
Problem was too large a pressure drop when the main jets were lit,
which resulted in pilot light shrinking. They had to come back and
take a feed from the gas main, rather than running it from an old
gas lamp pipe.

Have you checked the pilot flame's size/shape against the servicing
manual? It might be that it's misshapen due to partial blockage,
and the change in airflow when the main burner lights is taking it
away from the thermocouple. Another possibility is the thermocouple
isn't mounted at the right place/height. Another thought that occurs
to me is the flue's blocked, causing the flames to drift into the
wrong place due to lack of airflow (and consequent risk of CO release).

--
Andrew Gabriel