Set Appliance State:

Mains Power Off
Gas On; Pilot On
CH Demand
Selector Switch in CH & HW position.
CH Thermostat to Max.
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Observation:

Run Central Heating for approximately two minutes
(dependant on heating load).
Slowly turn CH Thermostat down.
Does main burner flame modulate?