Depending on the actual powerline device you may get 2 sockets, but you can always add a simple unmanaged switch as a poster mentioned before.The simple cable to the shed may be the answer. although chnelling it cleanly may be an issue, whats the situation with length? ie - when will my signal start to degrade due to cable length?100 metres
You still have the telephone extension problem.
Well, I could run a extension from the phone socket in the lounge I suppose. Or get BT to add one when first fitting the line.