Why does my boiler make banging noises, lose the pilot flame and drop pressure mively when switched on?

It is either the boiler pump at fault or the thermistor which is fitted on the flow pipe of the boiler. The banging is because the water in the boiler is not circulating, so the boiler is only heating the water contained in the boiler which is turning too steam which is causing the pipes to rapidly expand and cause a knocking noise. You are losing pressure because the water in the boiler is evaporating as steam.