Set Appliance State:

Mains Power Off
Set Multimeter to AC Voltage Set meter to measure AC Volts
Timer and room thermostat (if fitted) calling for heat
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Measurement:

Measure voltage across terminal marked 4 white wire (this is the 8th terminal from the bottom of the main terminal block) and the neutral terminal, blue wire (2nd terminal from bottom of the main terminal block).