Set Appliance State:

Gas Off
Isolate Mains Power to appliance as per TOPs P4
Turn Hot Water Tap On
Wedge the ignition lead between the 2 spark electrodes as shown in the image
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Reset appliance as necessary (hold down both arrow buttons until display changes).
Allow the fan to run / ignition sequence to commence and observe the end of the ignition lead.
Can a strong consistent spark be seen between the end of the ignition lead and the electrode bracket?