I did exactly the same thing!!! (though I had been rejected first of all for law and then decided geog was for me). I got in touch with the head admissions tutor (Dr Donald McNeil) and basically told him my situation, told him my grades and UMS + unit marks for geog (my grades I think were the deciding factor), he then got back to me and said no problem and made me an offer the next day. It was pretty simple, you should have no problem.