In reply to Mountain Llama:
There is a free OS map app download and one you can buy. It worked fantastically with the gps but not so well from mobile phone signal. Would give you a 1:50,000 scale OS map and then a google style map at about 1:25,000 and smaller.
I thought it was a very good app but it didn't cache the maps as it downloaded them. So every time you turn it on it needs to be able to connect to the internet to download each portion of the map.
Which can be a bit of an issue when you're in the middle of nowhere.
The pure grid reference apps like the compass and grid ref ones are ok, I never really tried it so cant comment on the accuracy but the compass was crap.
Also they suck power like a sponge!