Set Appliance State:

Mains Power Off
CH Demand
Isolate Mains Power to appliance as per TOPs P4
On/Off Switch OFF.
Remove Flame Sense Electrode (1)
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.
Replace Sense Electrode.