Set Appliance State:

Mains Power Off
CH Demand
Gas On
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Allow boiler to fire for CH and flame to establish.
Turn off CH Demand from external timer.
Observe the flame as the boiler loses its switch live.
Does it extinguish straight after timer turns off or does the burner show beads of gas still lit?