Glowworm Compact 100E

I have a combi Glowworm Compact 100E which is about four years old. Same old story, cover ran out last week boiler breaks today!
The symtons are as follows.
With hot water tap running, boiler fires up at 1 bar Pressure increases to 4 bar then a few loud bangs within the boiler. Boiler stops, then again it fires up, pressure increases then the bangs again.
After that the reset light comes on.
I have noticed that the overflow pipe outside was dripping after the above.
I have read that it is probably the diverter valve diaphragm split or something.
The central heating system works fine.
Any ideas?
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