Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Remove left - hand side boiler case
Remove electrical connector to the gas valve

Measurement:

Measure for resistance across the two outer gas valve terminals 1 & 4.
Expected resistance approximately 54.0 Ohms.
Note.
Ensure a good connection is made as misdiagnosis will occur.