Finally got around to checking this out today. I checked the voltage at the plug on the compressor itself and this was at 12.4v so I'm getting a voltage to the unit at least.
I've also had the system regased due to the aircon not working. On the compressor, the outer ring is spinning but the centre piece (with the bolts on it) is static. It can be moved fairly easily by pushing it round with a screwdriver.
Also did the interior checks through the climate control panel and got a few expected values and a few wierd ones as below :-
Code 1 System Malfunction - .0
Code 20 Aircon Compressor (A/C Clutch) Voltage - 14.6
Code 22 Cycle Condition of AC Refridge High Press Switch - .1
Code 28 Engine Speed - 44 ???? Dash Rev Counter = 900ish
Code 29 A/C Compressor Speed - 0
Code 31 Display Check - 2 ???? supposed to light up all panel segments
Code 49 Malfunction Counter for Speedo Signal - -60 ??? Minus large 6 small 0
Code 61 Software Version - 106 (large 10, small 6)
Any guidance welcomed but I'm pretty certain that its the compressor thats shagged - I just don't want to buy a new one to find that its a faulty switch or something daft!
I cant say for definite as i'm not familiar with the system on these cars but, i'm assuming that you did the diagnostic read out with the engine running and the air con switched on as the compressor clutch says its 14.6V.
I would think the clutch reading shown is what the control unit is supplying on the wire to the compressor, it doesn't mean its getting there but the fact that you have tested and got voltage there suggests that the wire may be ok.
The fact the outer of the pulley is turning but the centre is not should mean that the clutch is not engaged to drive the compressor.
I would check that the engine earth is ok although I would expect other faults if it wasn't. Then if you definitely have voltage to the compressor and an earth from it and the clutch isnt engaged, there is a good chance the compressor is your fault.
As mentioned above I'm not familiar with the system used so my information is based on BMW's.