Hi Leaf,
I think you should give it a go. You will regret it if you don't try, and what have you got to lose?
I'm considering doing the same thing in about a years time. I think they ARE open to applications from graduates from other universities, but I think you have to prove that you've got a broader knowledge of your subject beyond what you've studied in your dissertation/modules etc. You would probably have to have an absolutely wicked proposal, so I would suggest you get some advice from the uni's you are applying to, and possibly from teachers you already know from your uni (if there are any that you get on really well with).
Do you have your eye on any supervisors in particular? Or is it more a case of you just want to study your subject further at a reputable university? (For me, it's a bit of both). :-)