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
Selector Switch OFF
Gas Control Knob fully depressed.

Measurement:

Measure voltage across LOWER terminal of Boiler Overheat Cut Off and Gas Valve body whilst keeping Gas Control Knob depressed.
Voltage measured should be greater than 8mV DC.