Set Appliance State:

Set Multimeter to AC Volts Set meter to measure AC Volts
Isolate Mains Power to appliance as per TOPs P4
Boiler mains switch On
Remove control display unit
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Measure the voltage across the red lead (live) and blue lead (neutral) at the rear of mains switch.