Set Appliance State:

Mains Power Off
Programmer or selector switch set to hot water only
Heating temperature selector to OFF
Hot water temperature selector to maximum
Ensure heat bank is not up to temperature (this can be achieved by running the DHW with the mains power to the appliance on and the gas to the appliance switched off)
Gas On
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Allow fan to run and pilot to ignite
Does the main burner cross light from the pilot?