Call up sky, tell them you're thinking of leaving, those magic words should get put through to the "retentions" team and the floor is all yours for negotiations.
May have been the case a couple of years ago but I know of at least three people who have gone down this route and SKY have just said bye bye to all three of them. Maybe they were unlucky but I wouldn't hold my hopes out for a new deal on this particualr score.
Good luck mate!