Hello Rkell
I recently had an aircon problem on my S1. It was a different problem to yours in that the blower fan would not run although the aircon itself did work at speed and provided cold air simply though forced air through the system. I was given some trouble-shooting advise by the company that made the aircon control unit for the car that allowed me to identify the faulty part and though I suspect it may not help you I pass it on as it is perhaps something to could try when it's not working.
Firstly I took off the top of the heater box under the bonnet to gain access to the blower fan. I disconnected the wires to the fan and connected a 12v battery to the fan side to make sure the fan worked; it did. I then connected a 12V tester to the other wires to test if there was current coming to the fan; there was. This then identified the problem as being a current limiter inside the fan control unit that is mounted behind the dash. Unit replaced (approx 250USD) and problem solved).
As said before I understand your problem is quite different, but you never know.
Cheers
