I had a ganders at this book. My maths knowledge is not great ... but there is a key:
For understanding things like this, providing you have a creative mind and a good grasp of the abstract nature of structures and mathematical principles you should be fine. My mental arithmatic is pretty poor by anyones standards, but I love reading about discrete mathematics because of it's abstract nature.
I wouldn't consider myself a very good mathematician in the numeric sense, but as a software developer I like to think I have a mathematics (or mechanical) mind.
I wouldn't worry too much about your numeric strength but more about the nature of mathematics and the personality of the concepts underpinning computer science. If you have a good programming mind, maybe try and enhance that with combinatorics/discrete/concrete mathematics (which, besides counting theory, in many cases avoids dry numbers).
I found my love for things like set-theory studying compilers, and I wouldn't want to sit my maths A-level without alot of cramming!
Give it a go, what have you got to lose?
(im 22 and in a similar situation to you)
Good luck
PS: I also found the video lectures interesting. You can torrent them from
http://groups.csail.mit.edu/mac/classes/6.001/abelson-sussman-lectures/