I have found the problem but not solved it yet
On the heater box on climate control versions there are 2 servo units on the r/h side which you can get at from the drivers footwell (rhd cars). Both are held in with 2 screws (something like a 7mm socket is needed). The upper one controls where the air is sent, the lower one controls the mixing of the air. I took them both off and swapped them round and the heat started to flow again, confirming that is the problem. However I would avoid taking off the upper one if you can (there is a plastic piece between the servo and the airbox which is a pain to get back in place).
Getting the lower one off isn't difficult beyond the normal hassles of working while upside down with your head in the footwell.
I have picked up a 2nd hand unit but not fitted it yet. As a short term bodge I took the lid off the old servo, took out one of the gears, fitted the servo and turned the last gear to have the heater on full, but the gear back in place and put the lid back on. I would rather have full heat than no heat at this time of year.
All the best