you wont be aware of this but sky have made it very difficult to switch a sky hd box to a, what is known as a FREE TO AIR box. Trust me I have switched 3 just recently all amstrads and all different ages. It used to be if you did a software update but now the only way I got it to work was by using a uk free to view card. I also switched the box from a dual feed to a single feed.
you should look underneath box and read off exact model, and when you switch it on u should go to services and make a note of software version.
and its tricky getting it to change, the older the easier.