Hey Dongleboo,
Did you get to the bottom of this? I test drove a 2012 Audi A6 Avant 3.0 TDI on Saturday and felt exactly the same judder that you experienced, worst I found in higher gears at slower speeds when the engine was under more load and lower RPM I could feel almost a misfire, seller said he believed it was due to a injector that he has agreed to change what was noisy when cold. Wondered if this fixed it for you? I have to re test the car before purchasing but would love to know if the injector solved it for you, would give me a little more peace of mind!