Set Appliance State:

Mains Power Off
Set Multimeter to AC Volts Set meter to measure AC Volts
Boiler ON/OFF switch OFF
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Measure the voltage across terminal marked L (Live) and terminal marked N (Neutral) on main terminal strip marked ST12 on PCB.