the central heating is working and the boiler stays lit, works as it should. if you open a hot tap while it is fired for central heating, hot water is delivered. if you open the hot tap when boiler not fired for central heating, it does not always fire, and you have to turn tap on/off several times to get boiler to fire. if this is the case, i would be looking at the diaphragm in the diverter valve. there may be a related problem with a restricted gas supply which would manifest itself by the hot water not getting particularly hot. the BG fixed price repair covers one defect only so beware. the diaphragm is easy on a 240 - one of the easiest repairs in the book.