Hot water OK, central heating not, one radiator warm?

My system (Glow Worm Fuelsaver F boiler) is working fine for hot water, but the central heating won't come on. However one radiator (out of six) is slightly warm when the hot water is on. Usually when first switching on the heating in autumn it needs the hot water to be on, and then switch on the heating, the first time it's asked to heat. This time, no response at all from the radiators apart from the one mentioned above.