Set Appliance State:

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

Measurement:

Measure the voltage across terminal 6 (marked AMBIENT THERMOSTAT) and the neutral terminal 1 (Blue wire, marked N) on the main terminal block.
Note:
If no room thermostat is fitted there should be a link wire connected between terminals 5 and 6 on the main terminal block.