As I said before, I'd be all for having a go at most things myself, but I've done quite a bit of this sort of stuff before, through previous jobs and my own DIY.
TRUST ME - if you are unprepared for this, and you end up making a bags of it, it'll be very hard to fix it, or it will cost you so much more to have hard, badly laid concrete removed, then do it all again. If you really want to give this a go yourself, I'd strongly recommend that you do ample research, take your time building the frame (or get a friend who is knowledgeable about this type of work and who has done it before to help set you up), and get the readymix delivered. It may cost a few quid more, but you'll get it finished quickly, and you'll save your back. Remember, the best person to do this would be a contractor, so do what he does, and save time and effort. You're really only saving on additional labour costs. Also, remember to build in a slight slope for water runoff away from your house, or whatever other places you do not want flooding. Good luck!