Set Appliance State:

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

Measurement:

Measure the voltage between the brown wire marked pin 3 on X9 connector at the PCB and a suitable earth point.
Mains voltage expected.