Tonight our boiler decided to ignore requests to heat our hot water, instead it fires up and heats the radiators, even though the central thermostat is telling it to not do so. It's a Worcester Bosch condensor 25HE, only 3.5 years old, and running fine until now. I've tried hitting the resetting switch and turning it off and on several times, but no joy. What is wrong and is this going to be costly?