Banging pipes

Hi - I have a pressurised domestic boiler system that seemed to be working fine before I had a house extension and several radiators added. The diverter valve failed once and was replaced and after this time, I the pipes bang quite loudly when the thermostat switches in to stop the boiler. The banging only takes place when the radiators are on - there is no noise caused in Summer when I only have the hot water side working.
The system seems to keep pressure well (it can be topped up from the rising main), but I periodically (perhaps once every month) have to bleed air from a few radiators (so air does seem to be getting in somehow).
Does anyone have any idea what could be causing this? I have had two plumbers in - one replaced the divertor valve, and another put alarger pressure overflow vessel in the system - both have made no difference!
Help Thanks Simon.
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