I have heard that the proposal is the most important thing, whether they let you in will depend more on whether it is feesible etc and if you are doing well in your MSc then don't worry because your references will say so. Undergrad is important though, so i'm not saying it won't matter at all, but i'd definitely say he proposal is the most important thing, given that you have a post-grad. And just apply now, rather than waiting there's no point delaying, i guess there wil be a form that you write your expected grade, or your references will write that you are expected a distincton.