I purchased Memory Map, which allows you a limited amount of installations across PCs, laptops and a mobile device.
The app for the mobile is free of charge.
I contacted Memory Map who gave me links on how to install it, and it worked fine. After copying the files onto the device as instructed, and entering the licence key I was able to get it to work.
Sadly it doesn't have 3D mapping, but it does mean you can carry the whole of the UKs mapping on a smartphone or tablet, and it can be used as a GPS unit with mapping, although I found it fiddly and difficult to fathom. It does have the benefit that mapping can be accessed offline.
If you are buying computer mapping it's worth getting Memory Map as you can install it in a mobile device as well as your PC and laptop. If you are looking for an app specifically for a mobile device there are probably better and cheaper ones out there.