Set Appliance State:

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

Measurement:

Measure the voltage between the 2nd terminal from the left under "ROOM STAT / TIMER" to the "MAINS IN" N connection at the mains terminal strip.
With no demand mains voltage should be absent.