Central heating will not heat water?

the boiler will not heat the hot water, when hot water is asked for alone, on my central heating, when c/h is on as well, the hot water heats normally, it's a gravity feed system, i cannot find a diverter valve,anywhere in the house, the tank thermostat has been changed, immersion heater ok, c/h system looks quite old, its an ideal rs460 boiler. Thanks, Al