Download View Ranger on any smart phone and you get one region of the 1:50,000 series OS maps to use in it for FREE. Works off-line and with GPS to show you precisely where you are, does all the stuff you'd expect like route tracking, markers etc. I'd still have a paper copy of a map if I was using it for proper navigation rather then just to play around with like I generally do, but it is quite a good app in my opinion. Uses a LOT of space on your phone thou.
you can download 1:25,000 series maps into it as well, you get one of these for free as well to try out.
One thing it does show with the GPS feature is how incredibly accurate OS maps actually are.