There's a neat little app called GPS2OS, which gives grid ref, lat/long, altitude, etc, but you can also switch to a grid as per the OS maps, with a little dot to show your position relative to the grid, and you can also select a satallite view with grid superimposed. It's 1:50000, but until ViewRanger get their iPhone app up and running, it'll do. ViewRanger have new funding and developement people on board to expand onto other mobile platforms. This is in their news info on their site, and they were advertising for someone to develop the iPhone app a while back, apparently.