....... It has coincided with the cold weather, when I've been turning the heater knob on to full (and similarly, I don't know what that actually does -- is it connected to some kind of servo motor to open/close vents, or is it a simple mechanical linkage ?).
Simple mechanical linkage Peter. The knob operates a push/pull bowden cable in turn operating remotely fairly common heater valve, controlling the coolant flow through the core.
