potterton boiler comes on makes a banging noise and switches off?

we have opened the silver screw on the pump and rotated the motor with a screwdriver and the pump seeems to be ok. We have checked the water in the little plastic tank feeding the system. We have removed bled each radiator which came out with a mug of water. The system comprises of a boiler, pump and divertor valve and one way valve. There is a valve connected to the big pipes into or out of the boiler - when i open it, only a couple of drops of water come out. Please can you help us to figure out why the boiler is switching off?