I did exactly this after the September exams. I failed SP7 by 3.5 marks on average (one marker failed by 1 mark, one by 6 marks)
I went through each question and highlighted in the answer scheme the points I thought I'd got, and then compared my overall mark for that question against my SAR breakdown. It was quite interesting actually, as I think most of my problem was in HOW I was saying things rather than WHAT I was saying. For some questions, I was pretty certain I should have gotten full marks but both markers hadn't score me full. So clearly my wording/communication of my ideas wasn't clear enough or I was missing particular key words etc.
I ended up coming out with my own self-mark as failing by 1 mark, so in line with one of the markers. Although most questions I differed in one way or another - for some I wasn't sure why I'd gotten any marks, for others I felt they had marked quite harshly. But it came out to a similar place overall.
For me it showed that exam technique was my weakness, as I was thinking of a lot of the right ideas but just not structuring/wording my answer in the best way to ensure the examiner was clear that I understood the idea. Hopefully identifying that and working on it over the past few months was enough to get me over the line in April.