My plan is to disconnect one of the two cables going into the socket (it is definitely part of the upstairs ring, not a spur), extending it to reach the new socket, and run the return back to the original socket.
I intend to use crimps + heat shrinks to extend the cable, and run the new cable at the back of the wall, under the eaves - there is a small gap behind the wall that I can just about manage to squeeze through.
The wall both sockets are/will be fixed to is made up of plasterboard with 100mm celotex insulation behind it. I'd like to leave at least some of the celotex behind the backbox. Would it be enough to simply drill a hole through the celotex, just large enough for the two cables to go through, or does the hole need to be much bigger (the socket is likely to be used with a hair dryer)?
Is there anything wrong with my plan?