Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to AC Volts Set meter to measure AC Volts
Gain access to PCB (use Insulation Mat as required)
Boiler On
SUMMER/WINTER switch set to WINTER
System calling for Heat
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Measure the AC voltage between M21 terminal on Modulator board connector and the Neutral terminal.
WARNING!
Be careful not to short out adjacent points with meter probe.