From having taught the BMAT Crash Course for the last 4 years, we've found that students almost never struggle with the content of the Maths syllabus. Everyone knows the formulae, and everyone knows how to do basic mathematical operations. The things that everyone struggles with are (a) timing, and (b) intuition. Unfortunately, those are also quite difficult to teach, but we've done our best to help you with those in the Training Temple.
Maths in the BMAT is a bit different to the sciences in that they're not testing your knowledge of lots of different topics and techniques (you'll hopefully notice that you don't technically need anything more than GCSE Maths to answer the questions), but they're trying to determine your mathematical fluency - how comfortable you are in using and combining simple techniques in a variety of ways to solve a problem.
You do, of course, need to know some basic material to be able to attempt the questions, which is pretty well documented in the Official Section 2 'Assumed Knowledge Guide' (more on that below). The hard part will be in developing your mathematical problem-solving skills, but luckily we'll help you with that too! There's a dangerous misconception about Maths - that it's this innate ability that you either have or you don't, and that some people are "just good at Maths" and other people are "just bad at Maths". The first step to becoming better at Maths and acing the BMAT is to understand that Maths is just as much about practice and hard work as any other subject. Every Maths question on the BMAT is within your reach if you adopt this growth mindset (and do lots of practice).
The Official Section 2 'Assumed Knowledge Guide'
In 2014, the Admissions Testing Service, in collaboration with CGP, published a free, online guide to Section 2. This 400-page behemoth contains revision notes (in the style of CGP books) on all the topics that you might be tested on in Biology, Chemistry, Physics and Maths. If you're unfamiliar with this excellent resource, you should definitely check it - here's a link to the step-by-step instructions for accessing it that the Admissions Testing Service have put together - http://www.admissionstestingservice.org/for-test-takers/bmat/preparing-for-bmat/section-2-preparation/
The 'General' section of the Maths Training Temple will contain tips and tricks that can be applied to a lot of questions. Topic-specific material can be found under the other sections.