Set Appliance State:

Set Multimeter to AC Volts Set meter to measure AC Volts
Selector switch set to heating & hot water
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Measure the voltage between the brown and blue wires on the right hand side of the terminal block
(i.e. after the fuses, as shown in the picture).