Be wary of ebay ones. i got a cheap set off there thinking I could save a couple of quid, and when they came they were for a left hand drive car because the 'spoiler lip' bit was on the bottom, not the top. I ended up taking them apart and swapping them over myself to get them to 'work' properly (aerodynamically). Havent had any issues since but it as a bit of a faff to swap them over.
Cant go wrong with the £23 dealership ones as Tim said.