We get hot water when we turn on the heating but not otherwise?

If we turn on the external heating thermostat, the central heating system works (boiler fires and radiators come on). If however we want hot water but no heating, we cannot get. This is a recent problem, everything was working before. We have the timer set to both heating and water and this is working (we get a red light) but it doesn't seem to be telling the boiler to fire. It is is glow worm boiler (not a combi) (sorry can't remember model)...
Thanks for any advice