Set Appliance State:

Mains Power Off
DHW Demand
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Observation:

Allow boiler to fire on DHW and flame to establish
Turn off DHW isolation valve under the appliance.
Observe the flame as the isolation valve is turned OFF.
Does it extinguish straight after (within 2seconds) DHW isolated or does the burner show beads of gas still lit?