In message .uk, Ed
Sirett writes

By which he means (in layman's terms)

Check that the HT lead is intact and has not broken, especially at one
of the connectors (the insulation is also crimped, so it's not always
obvious that it's broken

Check that the electrode is actually in the pilot flame and has not
corroded and burned away

lastly, I would add, ensure that the earthing is good - no resistance
(less than an ohm) from the electrode earth to the pcb and the main
earth

There is no cremated bumblebee sitting on the flame sense electrode.

I knew there was something I'd forgotten

--
geoff