Set Appliance State:

Set Multimeter to AC Volts Set meter to measure AC Volts
Isolate Mains Power to appliance as per TOPs P4
Boiler on/off switch ON
Gain access to PCB (use insulation Mat as required)
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) on the mains switch connector (rear L/H corner of PCB) on the PCB.