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
Gas Control Knob in Pilot Ignition position

Measurement:

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