From Durham's website, the requirement is AAA at A2 and a B in GCSE Maths... both of which are criteria that you'll fulfill (well, if you're predicted AAA, at least). Whether or not you've got a good chance, I don't know, I have no idea how competitive Psychology is. But I'm assuming that Durham would be your top choice, yeah? Having one "optimistic" application isn't a problem - what have you got to lose? You have four other choices, after all.