Set Appliance State:

Mains Power Off
Set Multimeter to AC Volts Set meter to measure AC Volts
DHW Demand
Selector Switch to Hot Water and Heating
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Test between mains neutral and bottom left connection of on/off switch (brown wire).
Is there mains voltage?