Set Appliance State:

Set Multimeter to Resistance Set meter to measure resistance
Isolate Mains Power to appliance as per TOPs P4
Disconnect 9 pin connector marked CN3 on the ignition PCB

Measurement:

Test 1:
Measure the resistance between the top (nearest to earth wire)
blue and brown wires at CN3.
Resistance will vary by gas valve but should be approximately 1 - 7kOhms.
Test 2:
Measure the resistance between the bottom blue and brown wires at CN3.
Resistance will vary by gas valve but should be approximately 1 - 7kOhms.
Test 3:
Measure the resistance between both of the brown wires,
in turn, to a suitable earth point.
Resistance should be greater than 1M Ohm on both wires.