okey dokey
I have just made the plunge..
i was going to go with sky's 'lite' (free) option but they wanted £60 activation fee, instead they offered the 'unlimited' broadband option at £7.50/month which is free for the first three months... so that works out at £67.50/year and that can be cancelled (i think at any time after 3 months)
they do all the donkey work of getting mac addresses off bt
so all in it is looking just about ok.. will update on how activation goes... i am sure it'll all go tits up somewhere but can't say that I am going to get all that excited about it
oh - and I am getting sky movies free for 2 months... as long as I cancel with 1 months notice (dec 7th) I won't have to pay any extra... so movies it is for christmas then!! 
Alternatively there is a 1/2 price movies and sports offer on at the moment for 3 months.
but then I guess you have to pay under contract the full price for 12 months?
No mate, channel package upgrades do not carry a contract. Just 31 days notice to downgrade.