Many thanks for the suggestions posted so far.
@ ninthace: I have seen OS digital maps in the past that showed contour lines, footpaths, etc, but they looked something like Open Street Map (but better). Definitely digital and differing from the paper maps. Perhaps things have changed since then?
@ ninthace and Gin & Platonic: I know of WalkLakes and OS maps on Bing, I have those bookmarked on my pc, but they are both websites.
What I was hoping for is something like the two apps I have on my old phone, one free and one about £7, which when opened show you an OS map centered on wherever in the country you happen to be standing at that moment.
They both have the disadvantage that there is no map if you don't have a signal, and while the paid one opens on 1:25k, the free one opens on OS 1:50k but when you zoom in to 1:25k it moves off centre so you lose your precise position.
Perhaps there is something to be said for the Windows Phone after all!