Set Appliance State:

Mains Power Off
Gas Off
Set Multimeter to AC Voltage Set meter to measure AC Volts
Boiler On/Off Switch OFF
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Measurement:

Measure voltage across the live terminal (brown wire) marked 2R and the neutral terminal (blue wire, marked 1Mp) on the main terminal block.