Set Appliance State:

Gas Off
Isolate Mains Power to appliance as per TOPs P4
Remove the burner assembly from the boiler

Measurement:

Inspect ignition electrode, sensing electrode and burner for general condition.
Thoroughly check the ceramic for damage and ensure the electrode is not distorted.
Check the electrode gaps are as shown.
Note:
Before reassembly check the integrity of all disturbed gaskets and seals and replace as necessary also upon reassembly all disturbed joints should be checked for gas tightness.