Set Appliance State:

Mains Power Off
Set Multimeter to AC Volts Set meter to measure AC Volts
Turn Hot Water Tap On
DHW temperature control to maximum
CH temperature control to minimum
WARNING! - Mains voltage is expected during test follow TOPs P4 live testing procedure
Mains Power On

Measurement:

Measure on the left hand switch the voltage across the thick brown lead ( Mains Live) and a convenient Neutral.
Expect Mains Voltage