Set Appliance State:

Gas Off
Isolate Mains Power to appliance as per TOPs P4
Turn Hot Water Tap On
Remove the lead from the spark electrode and slide the red grommet out of its slot in the boiler casing
Carefully position the lead as shown in the picture, a few millimetres away from the metal work
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Reset as required.
Selector switch to DHW position.
Can sparking be seen at the end of the lead?
Refit lead and grommet after test.