Although it is not a major job to replace a thermocouple you would be advised to contact a local CORGI registered gas fitter. Legislation does not permit non qualified persons to work on gas appliances no matter how competent.
There is a possibility that the gas valve on your boiler is not receiving the signal from the thermocouple by being interrupted via a safety device incorporated in the boiler.
Donald J. Ross
December 2007
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