Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to mVolts DC Set meter to measure DC mVolts
Selector Switch OFF
Gas Control Knob in Pilot Ignition position
Gas On; Pilot On

Measurement:

Light pilot and 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.