Worcester R28 HE Greenstar Condensing Boiler, Hot Water But No Heating?

I have the above boiler and have hot water but no heating. My thermostat is hard wired and seems to switch it off when it does occasionally work. If I open the central heating valve manually and "jam" it open it will then warm the radiators if I use the hot water enough causing it to refill the tank. I do not have a diverter valve only two separate valves in the airing cupboard for Hot Water and Central Heating. Any help would be greatly appreciated. Thanks