Set Appliance State:

Mains Power Off
Set Multimeter to AC Volts Set meter to measure AC Volts
CH Demand
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Boiler ON/OFF switch ON.
Measure the voltage across terminal marked Ls (Live) on terminal strip marked ST8 and terminal marked N (Neutral) on main terminal strip marked ST12 on PCB.