Set Appliance State:

Mains Power Off
Turn Hot Water Tap On
Gas On
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Allow fan to run and ignition sequence to commence.
Does burner ignite and establish ?
(Burner light will change from flashing to solid when lit and established)
Note:
You are unable to visually check if the burner lights due to the boiler having no sight glass, so you will need to listen to know whether the boiler sparks and burner ignites.