Set Appliance State:

Mains Power Off
Set Multimeter to AC Voltage Set meter to measure AC Volts
Boiler On/Off Switch ON
CH Switch ON
DHW Switch in INSTANT position
CH Temperature Control in max position
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Measurement:

Measure the voltage across terminal marked 3 and the neutral terminal (blue wire, marked 1Mp) on the main terminal block.