Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Remove ignition PCB from gas valve and make safe

Measurement:

Test 1. Measure for resistance across pin 1 and pin 2 of exposed gas valve connection.
Expected resistance approximately 1.7 Kohms.
Test 2. Measure for resistance across pin 4 and pin 5 of exposed gas valve connection.
Expected resistance approximately 2.8 Kohms.
Note.
Gas valve connection Pin 1 is nearest to the rear of the boiler