These aren't GHD's, but i've used both these and GHD's and they are just as good. These are normally around �70, so this site is a real bargain. If you don't like these ones, then the site has loads to offer.
http://www.buywiseuk.com/shop/details.asp?ID=htts
I nought some Hair Tools ceramic straighteners from eBay. They were brand new and boxed, and usually retail for �70. I've tried my friend's GHD, and mine are just as good as hers. I would look for the cheaper ones if I were you.