Mark - my storm chase friend in the US has recently bought the Canon 15mm fisheye and he's sent me some photos - it's a great lens. It's worth looking at, at least you can consider it, it's about £500 but it's quality.
http://photo.net/equipment/canon/15-fisheyehttp://www.the-digital-picture.com/Reviews/Canon-EF-15mm-f-2.8-Fisheye-Lens-Review.aspxhttp://www.fredmiranda.com/reviews/showproduct.php?product=26&sort=7&cat=2&page=1When I've got the rest of the equipment I want, I'll get this as a fun lens.