Hi
Yes, I'm afraid that the MAT is meant to be tough. The mathematical knowledge and techniques required to do the questions are taken from a syllabus roughly corresponding to the C1 and C2 modules from A-level maths, though the questions are set more variously than A-level questions. To give you a rough idea, those who were shortlisted for interview over the last couple of years got 55+ in the test. Those who were offered places averaged in the 70s. So don't expect to be getting the kind of scores you might be used to getting for GCSE/AS papers. As a couple of people said above, it's worth practicing.
Quite a few of the Cambridge colleges require STEP, so it's worth looking into that if you're considering Cambridge.
Hope that helps. Good luck.