This must obviously closely match engine coolant temperature as measured by the water temperature sensor that displays on the dash as I noticed the engine always starts when the display drops to approx 1/3rd full scale, irrespective of ambient temperature.
This is what prompted me to ask about a second, electric, pump cutting in to keep the flow going. Else wouldn't the engine radiator cool very much more quickly than the heater matrix?
All academic really as the system works really well, I just find it all fascinating!