Set Appliance State:

Turn Hot Water Tap On
Isolate Mains Power to appliance as per TOPs P4
Reconnect the Flame Sense Electrode Lead
Boiler On/Off Switch ON
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Observation:

Operate the Boiler with a full DHW Demand.
Depress the Reset switch on the underside of the Boiler for approximately 10 seconds.
Does the Boiler reset and the burner re-ignite?
Note:
The inner front case has to be in position for boiler to complete ignition sequence.