Gas boiler banging, then pilot light goes out.?

The banging is caused by the boiler overheating. This can be because the pump is not working, or the thermistor on the flow valve needs replacing. if the Boiler overheats, it will shut itself down. Put one end of a scerwdriver to the pump and the other end to your ear then switch the boiler on, and listen if you can hear the pump start to spin. Thermistor is not expensive to replace.