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Its a tricky fault to get to the bottom off. I'd recommend calling Glowworm for a fixed price repair. Last 2 I had needed pcb but can also be harness or display pcb and not very easy to check.Read more:http://www.diynot.com/forums/viewtopic.php?t=207296#ixzz2v6fjlhhw
We have had a similar thing in the past on these boilers where it has shown f12 but has actually masked a dhw plate to plate exchanger fault, expensive because the changing of the user interface (display) board proved to be not needed but was what glowworm suggested.
Glowworm guarantee to fix your boiler for a fixed fee of under £200
The PCB is £183+VAT
I think it will give you the best outcome as they will keep putting parts on it until completely fixed