Set Appliance State:

Mains Power Off
CH Demand
Selector Switch in CH/DHW Winter position
Heating Thermostat set to Maximum
Gas On
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Observation:

Following Ignition the Burner should go to full flame.
After approx 2 minutes reduce the CH Thermostat setting until the first 'click' is heard and the Burner will reduce to a low flame.
Continue to reduce the Thermostat setting further, the Burner should now go out and a second 'click' heard.