Its one of two things IMHO....
Either your heater flap actuator is stuck, this belnds hot/cold air to acheive the desired temp. Sometimes the motor seizes however I doubt this is really your problem.
It sounds like the good old dry soldering problem. There are a few threads here. To resolve it you remove the AC control unit and heat up the solder around the unit's PCB, it sounds worse than it is. A soldering iron can be got for a few pounds, cheaper than anything ALFA or auto repairers will charge. It appears the control wheels and the inlet sensor can be affecetd by this dry solder issue, hence the unit cannot regulate temperature.
I have a similar problem. My car blows hot air constantly, except on LO. If I select 16.5c, I have watched the actuator motor slowly wind the mixer flap to the HOT setting in a few seconds. So I need to do this procedure too, hopefuly will get it done this weekend and let you know.