Set Appliance State:

Mains Power Off
Set Multimeter to AC Volts Set meter to measure AC Volts
Selector Switch to Constant.
Heating Switch to ON.
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Measurement:

Measure Mains Voltage across the Boiler Thermostat Spaded Connection 2 (Brown wire) and the Main Appliance Neutral Connection.