Set Appliance State:

Mains Power Off
Set Multimeter to AC Volts Set meter to measure AC Volts
Turn Hot Water Tap On
Selector Switch in DHW Summer position
Hot Water Thermostat set to Maximum
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Measurement:

Measure Mains voltage across Selector Switch Grey wire and Neutral.