Just on 4 years ago that I had the entire climate control system out of the car, but my recollection is that there were 3 stepper motors (all the same) controlling the levers and flaps for the various airflow combinations, I believe there was 1 on the left and 2 on the right. All that business in my photo is on the right of the CC unit and controlled the flow to the face vents- with the lever out of it's track I had no air at the vents at all. Sounds like the warm / cool air mix is probably controlled by the one on the left. I would suggest that you remove the trim pieces above both footwells, get in there on your back with a lamp, have an assistant manipulate the heater and a/c controls and determine if there is one stepper motor or set of levers that does not move in any combination of control selections.

 

Thanks @Pod XF.

From what I've read, the left passenger-side temperature blend actuator is the first in the chain. If this isn't working, then none of the other vents will output warm air even with a PTC electric heater element/supplementary heater.

PARTIAL FIX!

Well I tried removing the glovebox and couldn't quite do it. I removed the torx screws top and bottom so it was loose, but there must be a few other things to do. I drove about 8 miles today and the glovebox was rattling around a little. The air con air was still cold. I got back in the car an hour later for a short 10m trip and and I couldn't believe it. Warm air was coming out of the all the vents. I drove it an hour after that and warm air was coming out of the left/right vents, the rear vents but almost none was coming out of the centre vents.

How has this happened? Obviously something to do with loosening the glovebox. Is it possible that the lever on the passenger-side temperature blend actuator has moved/loosened? I'm really not sure until I get the glovebox out but what a turnaround for a problem I've had for months.

 

If memory serves, the glovebox won't come out without taking a lot of other stuff out first. And none of those things will come out without taking something else out first. Every single piece of these cars, when you want to just quickly remove it, seems to have been the first thing that came down the assembly belt and had the rest of the car built around it! To remove the glovebox, you might find you have to start with the back seats.