Set Appliance State:

Mains Power Off
Gas Off
Set Multimeter to AC Voltage Set meter to measure AC Volts
Turn Hot Water Tap On
Boiler Operating Switch in HW position
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Measurement:

Measure voltage across connection 12 (Violet and Orange wires) and connection 13 (Pink and Black wires) on the upper connector block.
Measurement should be approximately 24 volts AC.