Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to AC Volts Set meter to measure AC Volts
CH Demand
Gain access to PCB (use Insulation Mat as required)
ON/OFF Switch ON
Summer/Winter button set to Winter
CH Temperature selector to maximum
DHW Temperature selector to minimum
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Measure the voltage across the purple wire and the neutral on the main terminal block.