I've had billing problems with virgin for months, even going back to when they were telewest. They charge a £10 monthly 'late payment fee' if you don't pay bills in time. They were charging me this even though I was paying by direct debit!
I always pay on time and even overpay some months but it still goes on. I would've gone to Sky ages ago if virgin hadn't given me a monthly discount, I get about £15 off my bill every month, listed as a 'loyalty refund' So I get phone, 4mb BB and the medium TV package for roundsabout £32.
Apparently Virgin Media is up for sale, they've only been around for about 4 months!
Your best bet is to phone up sky and threaten to defect to virgin, pointing out the packages where you could get better deals. They will probably match or even beat the Virgin equivalent. Make it known that you don't care for Lost or the Simpsons as they might use Sky One as a bargaining tool.
You can definitely negotiate though, these companies would rather give you money off than lose you to a rival.