You could buy a bare box from:
http://www.simplydigital.co.uk/acata...Receivers.html
They can supply it with or without a Quad LNB (you might already have one if you have multiroom) and you would need to run a second cable from the dish to wherever the Sky+ box would be.
An alternative would be to buy the bare box and get a local installer to change the LNB and run the cable.
Once its in all you need to do is call Sky and get them to activate the recording features on your new box.