Let me put this nicely: you should aim for far higher than a C. It's done on UMS, which I think other people have already told you.
I'm not sure how exactly the A* works if you are doing S2 as an A2 unit, as I'm doing AQA maths at the moment and only C3 and C4 count towards the A* as they are the only two A2 units (I'm also taking S2, but I'm taking this as a part of further maths), but you should be able to achieve an A* if you retake S1.
S2 should give you a bigger understanding of the topic of statistics as the whole, so if you are able to understand S2 you will likely do far better in S1 if you retake it. You seem to be able to understand C1 and C2, so you should do well in the other core modules, and C3 and C4 are not too much more difficult than the first two core modules.