Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Remove the ignition electrode from the burner door

Observation:

Inspect the spark gap.
It should be 3-4mm from the tip of the electrode to the tip of the earth probe.
Note:
1. Before reassembly check the integrity of all disturbed gaskets and seals and replace as necessary.
2. Upon reassembly all disturbed joints should be checked for gas tightness.