The MSc (Econ) will move along at a very fast pace. The "September Course" in math and statistics can only refresh the math you already know. If you are weak in this area, which is very unlikely, you will be surprised at how fast they will stretch you. It will be exhilarating to study so much at such a high level in such a short time. If you are somehow weak in maths though, you will be miserable there, at least in the Economics program. As you know, math is what economics is all about.