there are lots of different types of bags. I just recently purchased a lowepro slingshot 200 and im very happy with it. It is a back pack but with only one strap so it slips over your body. it is designed for quick access to the contents while the bag is still on your back. Im very happy with it.
This bag comes in three different sizes, 100, 200 and 300 each being bigger and able to hold more kit. I would recommend the 200 as its a good size.
Thanks, I think I'll go for that one, looks pretty good.