Set Appliance State:

Mains Power Off
Mode selector switch to Off/Reset position
Hot water temperature control set to maximum
Ensure there is a demand for preheat - this can be achieved by running the cold water with the gas to the appliance turned off
Gas On
WARNING! - Mains voltage is expected during test follow TOPs P4 live testing procedure
Mains Power On

Observation:

With no demand does the boiler fire?