Money down drain - help please


I have a central heating problem that has not been cured for 3 years and involved 3 independent plumbers. Thermostat has been replaced in the boiler - pipe work re routed around new cyliner and today the system has been flushed out with air pressure machine. The conclusion of todays plumbers is that all the pipe work needs to be replaced
The symptoms - these are a bit eratic but the first sign is the clanging / banging pipes - header tank overflows as the excess heat is discharged into the tank - sometimes steam then followed by an overflow of water, The noise reaches a pitch that the boiler has to be shut down as the whole system feels dangerous.
Having had three different opinions and spent out a fair bit of money but no lasting solution I am anxious not to spend out over 1200 pounds for new pipe work to find I have the same problem but less money.
I am persuaded that it is an air lock causing the problem but it is weird as it can work well for a week and away we go again - false hope.
If anyone has any ideas or thoughts - suggestions get rounds or diagnosis it would be much appreciated
Thanking you in advance Keith
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is this a low mass gas boiler? Is this a solid fuel system? Need more info on how it is piped. It sounds as if it is overheating and not ramping down the flame or shutting the burner off at all. can we get a picture of the pipes around the boiler?

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