Set Appliance State:

Gas Off
Set Multimeter to AC Volts Set meter to measure AC Volts
Isolate Mains Power to appliance as per TOPs P4
Remove Terminal Block Cover.
Room Thermostat calling for heat.
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Measurement:

Measure Mains Voltage across connector 1 Black wire and Blue wire on the main Terminal Block.