Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to AC Volts Set meter to measure AC Volts
Ensure there is no demand for either CH or DHW
Preheat OFF
Gain access to the boilers main terminal strip
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Measure the voltage to Earth on the Frost input connection (far right, black and white wires) of the boilers main terminal strip.