Average marks in pre-honours, high marks in honours: do I stand a chance at Oxbridge?

Hi all, first post here!

I'm an applicant to a few politics/IR MPhil programmes at Oxford and Cambridge. Fourth (and final) year student at a Scottish Russell Group university reading a joint honours degree in a modern European language and politics.

Not sure how common my situation is, but I achieved average (~65%) marks in my first two years at university, with one C in both my first and second years and only one A (which I earned in my second year in a politics course). I became very serious during my third year, however, which I spent abroad at a reputable political sciences institution on the Continent. I came out of the year with an 83% average, and am currently sitting on a fourth year average of 74%, with a very realistic prospect of graduating this summer with a first.

So, my question is: how important are pre-honours marks in the choice of whether to offer a candidate a place? I'm confident in my application materials (personal statement, writing samples, recommendations) but a little worried about those two Cs and all those Bs in my pre-honours years.

Does anyone have any experience in this area? Thank you!