Official Apple cable. I know you can get unofficial cables for less but some people like buying official so for people wanting the official cable, this is a good deal, no?
£15 on the Apple website. Same on Play.com. More than that on Misco and Expansys.
You can possibly hunt for a while online and find it a tad cheaper but this is at Tesco Extra (Durrington, probably nationwide) so you can pop in to get one whenever you like and not have to wait. Hurrah!