Im no expert but,
The little box that sits out from your sky dish is called an LMB you get single, dual and quad ones, for sky+ you need a minimum of a dual lmb one line for the channel your watching and one for the channel thats recorded, i think, that the skyHD comes down the same cable, not sure i dont have it.
We have sky+ and i got the sky guy to put in a quad lmb with the view to "maybes" getting a second sky+, I ran cable from one of the spare unused LMB sockets to the old sky box and can watch free sky stuff (where i live there is no digital freeview reception).
Hope that helps a little.
you can get wireless tv transponders that will spit the output (non hd i think) from b&q etc, so that you can watch the same as whats on the main skyHD box, and the tv remote signals will be broadcast back from the box in your bedroom or wherever to the main box, but it means both TV will be showing the same so your wife canny watch corrie while your watching ray, you wont need your old sky box for this.
Like i said im no expert but have played around a little, id see if you have a quad lmb or not first. and how many cables are coming out of it.
Stoo.