Thanks Dan, I am now certain that it was a scam by the insurance co. to sell me a new boiler etc etc. Not happy and have requested a full refund of all premiums.
All fixed (I hope) pump changed and the boiler working fine on both HW & CH, no more banging (overheating).
FYI the circulating pump is required when you demand HW!
Many thanks for your help and advice, much appreciated.
Thank you all very much. Waiting to hear when the next "engineer" is coming...
So, when the boiler is cold/nothing running, is the hot water "pumped" round the boiler when you turn on a hot tap?
In the mean time I'll check supply to the pump...
I appreciate it's difficult to diagnose and also appreciate your help and advice too. The first engineer checked the pump and said it was fine and that the pump isn't involved/needed for the HW anyway (not my words, his)
I suggested "pump" to the third engineer and he poo-pooed that saying...
I'm running out of patience, 5 different engineers and its still not fixed. (Insurance cover)
The boiler is 18 years old but has virtually been rebuilt. New gas valve, new water to water heater exchanger, new diverter valve assembly (inc. new style filling loop) new expansion tank, new...