Hi Mike
I agree with Pete, changing the thermocouple is cheap and sounds likely to solve the problem.
If you have loud bumping and banging noises coming from the boiler though I Have the same boiler and was told that the boiler should have a permanent mains supply and the pump should be wired up as per the diagram in-side the gold pull off cover. This is to allow the boiler to run the pump after the heating demand has stopped to cool the heat exchanger.
Martin
March 2008