Hi, I've also got a 2002 A6 TDi and had a problem where the air con didn't come on. Like you I tried re-gassing etc, and was quoted £900 for a new compressor from an Audi dealer. I traced the fault to the rubber 'clutch' between the pulley wheel & the unit. I guess the compressor must be stiff and the additional load had caused the rubber to perish, so the pulley turned without turning the compressor.
I bodged mine up to test, by screwing 2 bolts through the pulley wheel to mate up with the shaft on the compressor. It does squeal sometimes when starting, but the air con works - and it's far cheaper than £900!
Hope this is of help
