I wouldn't recommend fitting a K&N or any "oiled" filter to a TD4. I fitted one to mine, and it made no discernable improvement to performance. In fact, over a period of a few months, performance actually got a lot worse. I did a bit of research and came to the conclusion that the oil from the filter had contaminated the MAF sensor. Problem resolved with a Roverron MAF compensator, but could perhaps have been avoided if I'd known better.
My TD4 regularly turned in around 40MPG - a figure my "new" V6 can only dream about ( ;)). Mind you, with petrol being about 15p per litre cheaper than diesel ...........