Set Appliance State:

Gas On; Pilot On
Isolate Mains Power to appliance as per TOPs P4
Link out hot water thermistor (orange spot - see (1))
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Observation:

Turn on HW Tap.
View flame through observation window.
Does Boiler light up?