Set Appliance State:
Gas On; Pilot On
Set Multimeter to mVolts DC 
Isolate Mains Power to appliance as per TOPs P4
Measurement:
Ignite the pilot by depressing the gas control knob on the gas valve and producing the spark with the ignition button.
Keep the gas control knob depressed and measure the voltage across the thermocouple interrupter wire at the gas valve and the boiler chassis.
Keep the gas control knob depressed and measure the voltage across the thermocouple interrupter wire at the gas valve and the boiler chassis.
Isolate as per TOPs P4
and refit as necessary