The black skirt underneath the bumper do sometimes appear on ebay for around £60. Seem to remember they are a bit more than Val quoted and near £140 from nissan.
I have a spare one which is scuffed but not to bad and hardly noticeable if yours is split or unrepairable. I swapped mine for a new one purchased of ebay. I can take some photo's of it if your interested in making a offer.
It's fixed with about six little bolts/clips. The undertray also clips into to it so slightly fiddly to put back on again but very do-able.