Just fixed exactly this problem on my 2001 A6 Allroad this weekend - it failed the MoT on a sticky door handle! Take door card off (easiest one I've ever has the pleasure of doing - two small screws at top and then lift up and off). I didn't bother to disconnect wiring, just tied card up to avoid straining the cables. Find the rod that comes down from the door handle to the lock. A small hand mirror to look into the depths of the door will help immensely. It is threaded at the end, but instead of trying the turn the plastic collar it fixes into, you will find that if you pull down on it (towards the bottom of the door) it will slide down. Then pull the threaded rod out of the groove and reposition - you will find that the rod moves down to open the lock so if the handle is not opening the door move the rod up in the groove. Push collar back up tight and you should find door handle works like it should! This is a very fiddly job and needs some patience, and skinny arms if possible, but well worth the effort, saving a £300 bill at a dealer!