Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Disconnect the ignition lead assembly connector from the spark electrode and from the PCB

Measurement:

Check resistance between each end of the ignition lead.
Zero resistance expected.
Check lead in its entirety for good condition and that it is not shorting to earth especially where it passes near the boiler casing.