Set Appliance State:

Mains Power Off
CH Demand
Heating Temperature Control to Maximum
Gas On
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Observation:

Allow the burner to ignite, when the boiler has reached a reasonable operating temperature turn down the boiler CH temperature control.
Does the flame picture reduce as the boiler CH temperature control is turned down?
Note:
It may be necessary to reduce the heating load by turning off some radiators to reduce time taken for this test.