Set Appliance State:
Gas On; Pilot On
Set Multimeter to mVolts DC 
Isolate Mains Power to appliance as per TOPs P4
Selector Switch Summer position
Gas Control Knob in fully depressed position
Selector Switch Summer position
Gas Control Knob in fully depressed position
Measurement:
Measure voltage across 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 8 mVolts DC.
Voltage measured should be greater than 8 mVolts DC.
Isolate as per TOPs P4
and refit as necessary