Set Appliance State:

Mains Power Off
CH Demand
Turn Hot Water Tap On
Isolate Mains Power to appliance as per TOPs P4
Remove Sense Electrode (1)
Boiler ON/OFF Switch OFF
Gas On
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Observation:

Observe the Burner status through the sense electrode access hole.
Switch on the Boiler at the ON/OFF Switch and allow it to execute its purge sequence.
Does the Burner ignite?
Note: The Burner will go out shortly after it ignites because the Sense Electrode has been removed.
Mains Power OFF.
Replace Electrode.