Alpha cb24 boiler no heating but hot water ok?

I have an Alpha CB24 combi boiler. The heating fires up for between 10 seconds and 3 minutes the cuts out keeps firing up every few minutes but cuts out and obviously isnt heating the radiators before cutting out. The pump is running ok and the thermostat is working ok. Anyone got any ideas as it is christmas and the wife is fed up of a cold house! (she didnt take too kindly to the suggestion of another jumper!!)