I got mine from Rose, as they were the only people with the 2011 versions in, back in November, also they are the folding version, most of the ones you see in the UK for the same price are steel bead.
Although Rose managed to make a mess of my last order, would order from them again, they currently have free postage, and seem to be generous with sending out vouchers.
For going off road on a 23mm, I only did this as I ended up on a trail I couldn’t get off, would want at least 28mm if planning for any dirt, 35mm+ if carrying panniers.
If looking for something cheap, what about these http://www.on-one.co.uk/i/q/TYSCSAMSW/schwalbe-sammy-slick-wired-tyre @ £9.99
Another good place to look and get some ideas on what people have used successfully is http://www.crazyguyonabike.com/