On the link above, ignore all the costs mentioned, if you already have the dish and box it only costs you �20.
In the FAQ it says
Q: I already have Sky digital satellite equipment - what else do I need?
A: All you need to gain access to the free channels available on the digital satellite platform is a Freesat from Sky viewing card, which costs just �19.57. If you�d like to access the interactive services available, you�ll also need a working phone line. To purchase a viewing card please phone Sky on 08448 244 400.