Majority of problems with tumble driers are caused by a dirty burner it does not take a lot of dust to cause the flame although blue in colour not to be strong enough to reach and heat the flame failure electrode
the pilot is actually a vitiation device, but none the less i beleive needs to be lit or the pactrol will sit there waiting and not open the solenoids.
sorry if as you say you are now retired from the trade, all the information is there in the manual charts 3 & 4.
I cant give anymore info, someone else reading may take as being a license to attempt their own repairs
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