Set Appliance State:

Gas Off
Isolate Mains Power to appliance as per TOPs P4
Turn Hot Water Tap On
Gain access to inner combustion chamber
Selector Switch in Hot Water Position
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Reset appliance as necessary.
Approximately 2 seconds after the fan starts to run can sparking be seen between the two ignition electrodes?