It is very usual for students starting the mathematics MSc to feel rusty on some areas and/or have some gaps in their knowledge. You should use the diagnostic quiz to help you to work out where you may be rusty and/or have gaps. You should also look carefully at the individual module descriptions to see what you are likely to need for each module. Once you have identified possible rusty areas or gaps, the following links and resources may be helpful.

Once you are fully registered you will have access to the maths and stats study site which have more specific revise and refresh material for each module. The material below is free to access before you register and aimed at non-registered students who want suggestions for preparatory material. 

Please note that the mathematics in the linked OpenLearn courses is not sufficient for any of the MSc modules, but the courses may help for specific areas. Some of it is also not specifically necessary for any MSc modules (but the skills and techniques developed studying the material will be helpful). You must look at the taster material for either M820 (applied) or M823 (pure) to get an idea of the level of maths that is expected on the MSc. The material for M823 needs to be read in conjunction with the textbook Introduction to Analytic Number Theory by Apostol. 

All MSc modules except M836 and M838 (and some M840 topics) require a good working knowledge of calculus, such as that found in UG mathematics degrees, or in UG degrees with a lot of mathematical content. The best way to refresh your calculus knowledge is probably to re-work through whichever textbook you used on your UG degree, but the reading list contains some suggestions as well. Mathematics textbooks can be expensive, but since mathematics does not tend to change over time, second hand copies of earlier editions can be helpful for background reading (but for some modules it is important to have the correct edition of the set text).  

M836 requires a good working knowledge of linear algebra, and M838 requires a good working knowledge of abstract algebra. 

For M820, M821 and M833, studying Chapters 1-8 plus Chapters 13 and 14 of Mathematical Methods for the Physical Sciences by Mary Boas will give you a good foundation, or alternatively Chapters 1-16 of Advanced Engineering Mathematics by Erwin Kreyszig. The book by Boas is easier to learn from, but can be very expensive new. However, there are second hand older editions around that are just as relevant to learn from.