I've already part solved this problem. It took a while but I eventually tracked the problem down to the fact that the exterior isolation switch had been letting in (and collecting) water. This then was in some way making the switched live wire into a permanent live wire. However my question is would you have expected this circuit to trip with the existence of water in the external switch? Its a simple circuit, 16amp circuit breaker, Internal isolation switch to a wireless thermostat receiver, external isolation switch to the Combi grant boiler.
Many Thanks.
Many Thanks.