Set Appliance State:

Mains Power Off
User Control to Off Position
Set Multimeter to AC Volts Set meter to measure AC Volts
CH Demand
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Measure voltage across the red leads on the transformer terminal block.
The picture may differ from the position of the wires in the boiler but the test remains the same.