Most of the time just getting a freeview box connected to your existing ariel will work, if not then you will need a new digital ariel. I assume you are on the Sutton Coldfield transmitter? If so then i should imagine a cheap freeview box will work fine, thats a powerful transmitter and you are quite close to it.
An old digital skybox connected up to a sky dish will open the basic fta channels, but it will not work with a ariel for freeview.