Thanks for the replies.
The alternator is a genuine Bosch 0123 520 003, 90-150A as fitted by an Audi main agent just over twenty five months ago, so out of warranty then I'm afraid.
I was informed that the car was looked at this morning by a qualified sparkey. As soon as he connected the battery and cranked the engine two things happened. The alternator gave out a cracking grinding noise, followed rapidly by a plume of acrid smoke, at this point the battery was very quickly disconnected. The only time I've ever had an Audi do a similar thing was on the M6, when the alternator freewheel pulley seized solid. The acrid smoke in that case was caused by the serpentine belt destroying itself. I'm not certain that a clutched pulley is fitted to this car, if it was then I think it's safe to say that it's kippered.
The A8 owner now intends to source and have fitted a second hand part, just to get the car running, I get the feeling that it'll then be traded in against a Lexus. I've given him the 'phone number of a certain well known A8 breaker
I've also offered to beat any trade in price offered in order to save this low mileage beast, time will tell what happens. But, A8 number three could be on it's way
Then again, once he drives the car perhaps he'll decide to keep it.