Thanks to all who replied. My current set up is sky in the living room sky multiroom in son's bedroom, i have a digital cable coming from the dish going across the attic and down conduit which comes into the livingroom behind the TV, so i cannot get another cable to fit inside the conduit as there are other cables in there as well, as for doing it myself i was letting Sky do the install, but i hate cables on the outside of my house.
I had similar set up (although cables were outside). I upgraded to Sky + in the living room(and yes i think it is very good)and left my son with normal sky box in his room. As others have pointed out you will need an extra feed from the dish to your box. This will give you Sky + in the living room. Your sons will remain the same normal sky box. (if he has sky + too you will need another cable to his room) On my old set up i never bothered having the telephone line connnected to my box, to try and use some of the interactive services this time i agreed. Just want to point out, the engineers are contractors and paid a set price for each install, and for this reason they will take the shortest / ugliest route between the 2 points, so make sure you are there to direct them. I wasnt and took me 6 months of arguing to get them back to make a decent job of it. I also wouldnt pay the £70, ring them and tell them you want sky + and that you are not going to pay for it, you maybe supprised in their response.