I know Barry, aka PepperD used to have a MINI diesel with a K&N induction kit fitted, there are threads on the diesel forum talking about this somewhere, i'll try and dig them out. I think the power 'gain' wasn't noticable, but it did make it more roughtier!
Some links for you James.......
John Bagley has a K&N, check this out,
http://www.mini2.com/forum/mini-one-...45-tuning.html
Seems you might actually LOSE power? Check this thread!
http://www.mini2.com/forum/mini-one-...rs-mini-d.html