Set Appliance State:

Gas On; Pilot On
Set Multimeter to mVolts DC Set meter to measure DC mVolts
Isolate Mains Power to appliance as per TOPs P4
Disconnect thermocouple at gas valve
Connect negative probe, using a crocodile clip, to the disconnected end of the thermocouple
Gas Control Knob fully depressed position

Measurement:

Whilst keeping Gas Control Knob depressed.
Measure voltage across thermocouple and suitable earth point.
Voltage measured should be greater than 12 mVolts DC.