Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Gain access to PCB (use Insulation Mat as required)
Remove boiler casing
Disconnect flame rectification lead (rear of the three electrodes)
Disconnect connector LV1 from the PCB

Measurement:

Inspect the entire length of the rectification lead, check the connectors are in good condition and that the wires are not damaged or kinked at any point.
Measure the lead resistance, green terminal (LV1) to electrode connector.
Expected resistance continuity (zero ohms resistance).