If you lose all the power in the house when this fault occurs it will very probably be the Earth Leakage Breaker more usually called RCD (Residual Current Device). This will trip disconnecting all household power if it detects a difference in current (usually 30mA or .03A) between Live and Neutral - the "leakage" between the two assumed to be running to Earth and possibly through a person.
Something electrical in your boiler is faulty and is very probably leaking mains current to Earth. The most probable culprit (as a guess) is a solenoid coil but it might be something else.
A qualified boiler engineer may be able to help. Some of them tend to guess. Given that you will be paying for skill and expertise in diagnosis be prepared to challenge what he says. Tell him you will not pay him for guessing.
Whatever it is it will be just one failing part so don't let him get away with changing several things at your expense - boiler parts are often ludicrously expensive.
Peccavi, February 2009
