Viewranger App….download 1;25000 OS maps as needed in 5km square grid “tiles” (app comes with credit to buy tiles and top ups available “in app”) so no need to fork out£90.00 for the whole country.
Mount in waterproof case….best find on my internet hunt and brilliant in practical use is this…. http://www.cyclewiz.co.uk/BikeConsole-Bike-Mount-for-iPhone-4S-4-3GS-3G_AY5CC.aspx?0
….allows rotation to landscape and handles cross country off road well with the supplied mount. Probably not ideal for full on black route trail centre but then why would you need it then anyway??!
Best feature of the viewranger app…..you can mark out waypoints on a chosen route and then with it handlebar mounted the GPS dot shows you where you are on your route with the route a series of coloured lines(customisable in colour/thickness) along your waypoints. You can export or import routes as well. Turn off wifi and bluetooth, tone down screen brightness and get 4 hour+ rides on a iphone 4. Turn off screen lock and tone down brightness further and you have a brilliant night ride always on map with live location update! Run it alongside an app like cyclemeter and you get to track speed, distance, altitude, and route on map + ability to export, give live tracking info, track competitors etc.
I have been able to chart out night rides in completely unknown countryside near where I live, on and off road and discover lots of new territory without having to stop start to look at conventional maps to work out where the hell I am. Result.