As has been said, K&N filters or any other oiled filter aren't any good for the HDi, the oil on the filter element seems to be very good at killing mafs. The standard airbox is excellent, just buy a decent quality oem filter for it. My car actually felt noticeably slower with a top-end BMC CDA enclosed filter than it did with the standard airbox, so i can't imagine a £20 cone will do better.
Also, midbox to backbox conversion is a good idea for better exhaust flow. There's various threads in the guide section covering stuff like this.
Also, midbox to backbox conversion is a good idea for better exhaust flow. There's various threads in the guide section covering stuff like this.