Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to mVolts DC Set meter to measure DC mVolts
Pilot lit.
Set multimeter to 300 mv range.
Gas On

Measurement:

The thermocouple is terminated on the gas valve via a plastic connector.
Measure dc mv across thermocouple between this connector and the casing of the gas valve.