Comet are currently doing Sky HD for £99 + £30 installation as long as you sign up to Sky Talk (which is free).
I phoned Sky today and they told me that if I wanted to upgrade to Sky HD they would do it for £249.99 if I took Sky Talk or £169 if I took Sky Talk and Multiroom, neither deal really interested me.
I then saw in Comet earlier their deal and it beats Skys own deal hands down.
From looking at my receipt it seems that the £99 goes direct to Sky and the £30 so called installation fee actually goes to Comet so I wouldnt imagine you will be able to haggle for free installation!
As I stated you have to sign up to Sky Talk which is free and this provides you with free evening and weekend calls (BT Line rental still payable). Also you have to pay £10 p/m Sky HD subscription which is standard.
Visit either your local Comet store or I imagine you could call the sales centre.