Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Remove the spark electrode assembly

Observation:

Thoroughly inspect the spark electrode assembly for damage, cracked ceramic and for correct spark gap.
You may have any of the types of the pictured electrodes depending on the boiler serial no.
All types of electrode are satisfactory as long as the gap is 3.5mm ± 0.5mm
Refit spark electrode assembly after test.
Note:
Before reassembly check the integrity of all disturbed gaskets and seals and replace as necessary.