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).