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 upper pins marked V2.
For gas Valve A resistance should be approximately 140 Ohms.
For gas Valve B resistance should be approximately 78 Ohms.
Reconnect plug after test.