Thanks, polesapart, for the clarification.
The fully-qualified fitter who looked at this for me was from Hereford Gas, and is familiar with my boiler having serviced it several times previously. Having checked the piezo, its leads & connections, he explained that the plate (mounting plate) had...
I have an ancient (30+ yrs) Myson Apollo boiler which still works except for the spark ignition, where the small earthing plate plate which the igniter arcs to, and which sits by the gas flame, has (not surprisingly) disintegrated due to heat and old age, and is (also not surprisingly) no longer...