Set Appliance State:

Mains Power Off
Set Multimeter to AC Voltage Set meter to measure AC Volts
CH/HW switch in HW position
Ensure that the boiler is cold and ready for pre-heat
Boiler On/Off Switch ON
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Measurement:

Measure mains voltage across one of the black spaded wires at the boiler On/Off switch, and the neutral (blue) on the mains supply connector block.
Repeat this test on the other black spaded wire on the boiler switch.