Am already a Sky customer, and when I put in my details in the Compare The Market broadband checker I got the following deal offered:
Download speed up to 38 Mbps Fibre
Upload speed up to 10 Mbps
Monthly usage 25 GB/month
1st year cost FREE for 12 months then £10.00
+ Line rental £17.40
12 months contract
So it would appear that it is 12 month contract and you only pay the £17.40pcm line rental for the first year. The catch seems to be a 25GB monthly usage cap (needs verifying), although confusingly it also says "Traffic management policy: Unlimited and Lite broadband are not subject to traffic management. Sky Broadband Connect is subject to a Network Management policy". Still, seems an attractive deal for anybody that isn't a heavy user.