Right, mine is now fixed with the said part. £14.98 from you local Audi dealer. The only trick is that ElsaWin doesn't tell you how to replace just the check rod. It does however tell you how to replace the whole door arrester! But you don't have to replace anything on the A-pillar to fix it, as long as you can release the locking pin covered by a piece of black rubber.
If you have doubt removing the locking pin without damaging it, get yourself a spare one 8Z0 837 257 as you order the check rod, can't be that expensive.
The simple procedure is
1. Remove door car as per ElsaWin (or search the forum)
2. On the A-pillar, lift the rubber cover on the door check rod to reveal a locking pin
3. Use a plier wrench (or gentle hammering/tapping) to push the locking pin from bottom. Don't try to clamp across the pin top and pull, you may ruin it as it is hollow.
4. There are two notches at the top of the locking pin which you can also utilise to lever it out.
5. Check rod is now free, unscrew two hex nuts on the door to release the whole part.
6. Put the new one in and reverse the above procedure!
should be done in 30min or less