This has been an on-going thing for years with Virgin, NTL and Cable & Wireless. There have been many threads about it.
There don't seem to be any sence in it. I live in one part of Peterborough and can't get it (though I was promised it in 6 months many years ago), but someone in a different area not 5 miles away can. And both of us have the same prefix code.
So work that out.
