I've just been to my local Morrisons and Sky have a stand down there and are offering free installation and a £20 Morrisons voucher on payment of your first bill. That makes initial outlay about £70 cheaper! I'm sure its not just my Morrisons, so you may want to 'think on!'
I also plan to ask for more discount if I recommend a friend - in for a penny in for a pound as they say!