Set Appliance State:

Gas Off
Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Boiler ON/OFF Switch OFF
Disconnect the multi-plug connector from the Gas Valve

Measurement:

Measure resistance across the two lower pins marked VI.
For gas valve A resistance should be approximately 70 Ohms.
For gas valve B resistance should be approximately 78 Ohms
Reconnect plug after test.