Set Appliance State:

Mains Power Off
Gas Off
Set Multimeter to AC Voltage Set meter to measure AC Volts
Turn Hot Water Tap On
Summer/Winter Switch set to Summer.
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Connect multimeter to pin 26 (+ve) and Neutral.
Turn on HW tap and measure voltage.