In an earlier contribution to this discussion,
MARTINHOG wrote:

Hi,Discovered that capacitors C12 and C13 (100uF) had failed and that
C20 was about to die. Changed all 3 and now it works a treat !.

Only do this job if you are experienced in PCB component replacement,
the PCB is at mains potential.

You surely didn't do this in situ, with the boiler/PCB powered up?!
--
Cheers,
Roger
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