Dave
Firstly it is illegal for you to work on a gas appliance without a Corgi Cert so call in a Corgi.
Sounds like a blocked heat exchanger, I assume that this is on the hot water to the tap you get the banging and if so its probably scale. I am pretty sure the water aint getting through fast enough and is boiling off locally. Need to get a man who can mate sorry
Kenny F
October 2008