My Central Heating wont work! (Ferroli Falcon combi boiler)?

First off, we have a Ferroli Falcon combi boiler and no manual...
Current pressures are 1.8 when off and 2.1 when on.
Hot water still working perfectly.

For the past week, our central heating hasn't been functioning properly. When switched on the green radiator light comes on for a few seconds and then (seemingly as soon as the fan starts up) the red 'waiting' light comes on. Thus it does nothing... Every so often the green light will come on but then go off again as described above. About 40% of the time the boiler will start up but then instantly switch off as the red light comes on. Radiators only achieve luke warm at best.

Please help!?