In reply to estivoautumnal: I've been the proud owner of a 5D mkiii for a week now, bloody steep learning curve!
I boought and receieved, yesterday, a 17-40mm wide angle lens, I cannot wait to get out and use it!!
No real advice on purchasing it, as I bought the camera and lens from a guy here in Aus. I don't know if he delivers to the UK, but his service was perfect.
http://www.mycamerafocus.com.au/