Chris...
Theres no advanced maths or physics at all, its all reasoning and awareness testing.
As far as the 1st stage goes, if you get invited you usually get to choose when and where you attend, they hold them on different dates in a variety of locations (London, Manchester, Glasgow etc) and you choose one via a kind of email booking system. as i remember they then send you a pdf document of info to 'learn' which will contain all the stuff you need to know. as mentioned previously it is handy to know roughly how far certain places are apart, hard to be specific but general geographical knowledge will get you through that bit.
in my case i applied april 19th last year and was through the whole process in 7 weeks but i understand there may be longer times atm. i took my first stage on april 29th i think so 10 days time then...
any other q's pm me, but there is a lot of helpful stuff in this thread if u get time to read through it
adrian