Set Appliance State:

Set Multimeter to Resistance Set meter to measure resistance
Isolate Mains Power to appliance as per TOPs P4
Remove connector from the ignition PCB
Disconnect the lead from the gas valve

Measurement:

Measure the resistance between pin 4 (blue) on the
connector and pin 1 on the gas valve lead.
Measure the resistance between pin 5 (brown) on the
connector and pin 4 on the gas valve lead.
Expect continuity / zero resistance on both tests.
Also visually inspect the condition of the harness.