Set Appliance State:

Gas Off
Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to AC Voltage Set meter to measure AC Volts
Turn Hot Water Tap On
Boiler On/Off Switch ON
CH Switch OFF
DHW Switch in INSTANT position
Remove diverter valve microswitch cover
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Measurement:

Measure the voltage across the red wire on the two wire microswitch and neutral terminal (blue wire, marked 1Mp) on the main terminal block.