Set Appliance State:

Gas Off
Isolate Mains Power to appliance as per TOPs P4
Turn Hot Water Tap On
Boiler DHW thermostat set to MAX
Gain access to the electrode
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Ensure spark orientation is correct to the burner rail.
Isolate Mains Power to appliance as per TOP's P4
Measure the spark gap.
Spark gap should be 3 - 4 mm.