On Friday, 29 November 2019 16:43:15 UTC, wrote:

It was probably inadequate design or inadequate rating it was such a
common fault. You can actually buy, or could, identical replacement
transformers to avoid the cost of a new PCB. I repaired one that way,
but added extra ventilation around the transformer.

Yes, just bought one. One other company I contacted (YHDC in China) so they don't keep one as a stock item but would make me one in 3-4 weeks!

Another regular fault, is the fan and especially the wiring to to the
fan, which runs up the rear of the boiler. The heat from the boiler
melts the insulation.

That's not good to hear, but good to know - thanks!