Well I replaced the thermocouple last night and cleaned the jet on the
pilot light again. It took quite a while to get the pilot to stay lit
but the flame was much stronger than it had been previously. It did
work well for quite a while last night and did stay lit even after the
main burner had shut down again, but there was one occurance of the
pilot being extinguished again. Not sure when it happened as I wasn't
watching it constantly. I relit it again and it seemed to work after
that and was still working this morning even though the timer had
switched the heating off and then back on again overnight. I'm
keeping my fingers crossed but I'm not yet convinced I've got to the
bottom of the problem.