Set Appliance State:

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

Measurement:

Measure the voltage across the red wire (connection 4) on the switch PCB top multiplug (marked B) and terminal 5 on the main terminal block.