In reply to metrorat:
Very good experience of the Outdoors GB app on my iPhone 5s. The basic package for a few quid gives you all the National Parks at 1:50k. Large areas of OS 1:25k are reasonably priced, or there is the option of subscribing for a year which allows you to view 1:25k maps on your wifi at home, then access them offline when you are out and about. This works so well that I've found no need to download any OS maps at all. You can record your tracks, and plot routes on the Outdoors GB website (in full glorious 1:25k) , as well as download routes that others have posted (for free). My last subscription cost £10 which seems ludicrously good value.