Set Appliance State:

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

Measurement:

Measure the voltage across terminal 4 and the neutral terminal (blue wire) marked 1Mp on the main terminal block.
Note: If a room thermostat is not fitted a link wire should connect connections 3 and 4.