Set Appliance State:

Mains Power Off
Set Multimeter to AC Volts Set meter to measure AC Volts
CH Demand
External controls calling for CH
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Measurement:

Measure the voltage at terminal 11, marked SL (red lead) and terminal 10, marked N (blue lead) at the terminal block.
Note:
If no external controls are fitted ensure there is a link between terminal 9 (marked L) and terminal 11 (marked SL).