If you took out sky last December using the deal here: http://www.hotukdeals.com/vouchers/free-sky-hd-and-half-price-full-pac/537848
You should know that you are approximately 11 months into this service. In December (on your billing date) your bill is about to double (at least) - and they'll start charging you £10 for SkyHD too.
Sky also require 31 days (minimum) to cancel (and 40 days maximum) - you should also ensure that they set the date to cancel the day before your December bill.
I called to cancel since my pack was going from £25 (tho it started at £22.50 when I took it out last year) to £60. Guy sorted it out no problem - and strangely didn't even try to keep me as a customer.
I'm only posting this so you can either a) save your money or b) try and wangle a better deal out of them (there is another thread about at least getting 6 months half price and free HD for existing customers). I'll post the links in the next comment.