Set Appliance State:

Gas Off
Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Disconnect the plug (pink leads) at the 2 way Inline Connector situated on the L/H side of the appliance below the Gas Valve

Measurement:

Turn Hot Tap ON.
Measure resistance across the terminals at the inline connector (not at the disconnected plug). Resistance measured should be short circuit.
Turn Hot Tap OFF.
Repeat test. Resistance should now be open circuit.
Reconnect plug after test.
Note: Cold water inlet service valve can be used for this test if preferred