Set Appliance State:

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

Measurement:

With the DHW Tap fully open and the DHW Temperature Potentiometer set to maximum measure the DC Voltage across the Red wire and Black wire on the Stage 2 Gas Valve Solenoid (solenoid 3).
The voltage obtained should be approximately 24 V DC.