Set Appliance State:

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

Measurement:

Test 1:
Measure the resistance between the two outer pins on the gas valve.
Resistance should be approximately 4.1kOhms.
Test 2:
Measure the resistance between the top pin on the gas valve and a suitable earth point.
Resistance should be above 1M Ohm.
Test 3:
Measure the resistance between the bottom pin on the gas valve and a suitable earth point.
Resistance should be above 1M Ohm.