I have a pretty ancient potterton boiler which has all of a sudden stopped working. When heat is called for it is not firing up for hot water or central heating it is just making a pretty awful noise as if a motor is grinding. When I turn it off from the central heating control it goes off with a thump. The pilot light is on and I did manage to get it to fire up once after it had been left for a while. Does anyone have any idea what the problem could be?