sounds to me like your plate heat exchanger or temperature sensor especially if you dont have a scale restrictor fitted to the boiler or your heating system is dirty, british gas do a good value repair for 175 pounds but would probably try and push a power flush, id take the repair and get an independant installer to fit a magna clean directly after, ist going to be about 400 pounds for the two jobs, im sorry to inform you, but if it is the plate, which it usually is with the symptoms and boiler age you have described, then that is going to be the solution or a new boiler
john, January 2010