I know everyone else has said it, but I'd suggest first and foremost
you speak to Sky. It's not in their interest to rip anyone off in the way you describe, so chances are it is a a mistake (either on their part or yours). You will need to talk to them to get them to investigate what has happened.
If you truly believe you have been ripped off, I would suggest you contact your local Citizen's Advice Bureau. You can look up your local branch (and some advice on common problems) at
http://www.citizensadvice.org.uk/
Bear in mind, however, they are a charity. As such, the hours the branches are open are sometimes limited, so you may need to take a day off work to see them.
I can heartily recommend the CAB because they sorted out a problem I'd been having with Npower for months. Even if CAB can't help you with the payment, they may be able to suggest somewhere you can get the complaint resolved.
CAB may, however, insist that you talk to Sky first.
to the forums, by the way.