Icon 23-t Fires up and goes off. Just runs with no heat?

Hot water works but not very hot.

Icon 23-t.
Heating comes on but when the radiators are warm the burner cuts out. If left to run it only fires up for about 2-20 seconds at a time and the radiators are cool or cold. The place doesn't even warm up?? The bar pressure is between 1 and 2 bar, the radiators are full but the system was burning a lot of fuel prior to problem over the last few days.

Does anyone know what the problem is??