Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to AC Volts Set meter to measure AC Volts
Summer/Winter Selector to Winter
Ensure there is no demand from heating controls
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Measure the voltage across connection 'Room Thermostat 1' and connection 'N' on the mains input terminal.