Set Appliance State:

Mains Power Off
Gas Off
Set Multimeter to AC Volts Set meter to measure AC Volts
CH Demand
Rotary Switch to CH/DHW position
CH Thermostat To Maximum
Hold in Boiler Case Switch
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Measurement:

Measure Mains Voltage across Neutral (Blue) and Top Room Thermostat connection.