Isolate Mains Power to appliance as per TOPs P4
Remove the connector from the gas valve
Remove the screw completely from the gas valve connector
Reattach the gas valve connector to gas valve but
facing outwards as shown in the picture
Set multimeter to Diode test mode
(click here for how to do this)

Measurement:

Expect a reading of approximately 0.55V DC on each test.
Test 1: Red probe on pin 5 of CN1 and the black probe on the bottom pin of the gas valve connector.
Test 2: Red probe on the mains neutral and the black probe on the bottom pin of the gas valve connector.
Test 3: Black probe on pin 5 of CN1 and the red probe on the top pin of the gas valve connector.
Test 4: Black probe on the mains neutral and the red probe on the top pin of the gas valve connector.