Try to learn it all. The progress tests are designed to ensure you have a good general understanding of the material before the brush up course. Then when you get on the brush up the tutors will build on what you already know.
Some areas are tested more than others in the exams, but these change and the guys at Bristol will guide you on what seems to be the current "popular" areas when you go on the brush up. Don't try to second guess and miss something out as this will be the one thing that is guaranteed to pop up in your exam. There were some sections that I was never tested on but that is probably just the luck of the draw on the day.