Do you definitely need a new socket? You can buy wireless telephone extensions (that work with Sky etc) and obviously there are DECT phones that only require one socket in a house. You may have a very good reason but just in case you weren't aware of the above options
You can get the wired extensions pretty much anywhere as razorsharp said, they'd cost you about 10 quid, some you just plug in and some you wire in to the box.
The wireless ones I have seen in Maplin you plug in to your electrical socket then plug the second one into another electrical socket where you want the extension. They support sky apparently. There may be other types.