Hi mate, did you ever solve your Issue with the A6 Bitdi? I’m having very similar sounding symptoms at. Engine runs rough when cold, occasional injector knock, stinks of diesel and it stutters when you rev it from cold. Seems fine when warm and Audi have come back and said compression is fine (28-29 cold and 29-30 hot) and said injectors are balanced. Can’t find much online other than your experience. Cheers, Michael