Set Appliance State:

Mains Power Off
Gas Off
Set Multimeter to AC Volts Set meter to measure AC Volts
CH Demand
On/off switch in ON position
Mode selector switch to constant
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Measurement:

Measure mains voltage across connection 48 (Red/Black) and a suitable neutral
Expect mains voltage.