I think it was lack of spare parts that was the reason for not being able to repair it. Finding the correct differential is a nightmare, no one seems to know what they are talking about when I go to buy a used one, they quote the serial number as the 0ar******** number which is on the differential but this is only the differential case code, the important bit is HNQ (this is also stamped on the differential along with the date code) which gives you the internal gear ratio which is 4.1 in my case. No one seems to understand this even though I clearly state it when sourcing a used part. I had a differential sent from Germany, they confirmed everything was correct including the code but when it arrived it had HNR stamped on it which was incorrect so I had to send it back (the drive shaft flanges were 75mm dial instead of 70.7mm).
Finding one for my configuration of car, 6 speed manual, Quattro, 1.8t seems to be a rare combination.
I will try the links above as cars come in to breakers all of the time but as I said they don’t understand what is required.
Thanks for all of your suggestions and feedback.
Nick