Set Appliance State:

Mains Power Off
Gas Off
Ensure secondary heat exchanger is cool and requires pre-heat (this can be achieved by running the DHW with the gas to the appliance turned off)
CH switch OFF
Mains Power On

Observation:

Pre-heat switch ON.
Allow fan to run.
Can sparking be seen at the discharge tube on the ignition board PCB 22?
Note: Discharge tube is located on the underside of PCB 22 towards the right.