Will Exeter Uni let me in for maths with a grade lower in A Level than was asked for?

I think I'm going to get what I need in my other two subjects, but for maths they specifically asked for an A and I think I might end up with a B. How likely is it that they might let me in anyway? I also have extenuating circumstances (which are listed as valid ones on their website) which I disclosed on my UCAS form. And is there anything I can do to make it more likely they might let me in with a B if it does come to that? I really want to go there