Set Appliance State:

Mains Power Off
Gas On; Pilot On
CH Demand
Rotary Switch to CH/DHW position
CH Thermostat To Maximum
Hold in Boiler Case Switch
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Observation:

After a suitable period, slowly reduce CH Thermostat setting to a minimum, does burner flame reduce or extinguish?