Hi jeffn
The short answer is if you want to get Sky Broadband then yes you would need to switch back to the BT network, this can take time and be a fair bit of hassle but with the savings you make it might be worth it.
My advice is go back to NTL and threaten them that you're going to switch to Sky, by the sounds of it you could get your bill reduced by £40-50 a month if you say you're want to move to Sky - if they don't offer you a better deal then switch anyway.
Like i said it's a consumer's market at the moment and you're the only winner at the moment, don't let ISP's take advantage of you because you can get a far better deal than what you're on at the moment.
Cheers
Jason Lloyd