Set Appliance State:

Gas Off
Isolate Mains Power to appliance as per TOPs P4
CH Demand
External controls calling for heat
Remove the combustion chamber cover
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Observation:

Reset boiler as necessary.
Allow fan to run and ignition sequence to commence.
Can sparking be seen between the electrode tip and the burner?