How can I fix my gas boiler. It's an Ideal Classic fanned flue gas boiler?

The boiler is working to a degree, it heats the hot water supply to the house perfectly but will not heat the water for the central heating. If I turn off the hot water and try and fire up the central heating on its own then nothing happens, if I try and fire up the hot water and central heating simultaneously then the hot water comes on but not the heating. Obviously the boiler is working, it fires up without any problem and makes no strange sounds, it just ain't giving me heat. The boiler is an Ideal Classic lxff and ff fanned flue gas boiler.