Set Appliance State:

Mains Power Off
Set Multimeter to AC Voltage Set meter to measure AC Volts
Boiler Operating Switch / Programmer set to WATER only
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Measurement:

Measure the voltage across connection 2 (White wire) on connector X10 and connection 2 marked N (Blue wire) on connector X7 on the main PCB.