Identify a socket that runs through the cupboard.
Disconnect the cable from the socket and tape on a length of new cable.
Pull the old cable back into the cupboard, dragging the new cable there.
Terminate old cable for the switch.
Connect new cable to the switch and socket
Hadn't thought of that. May be worth a try but I suspect they are tied in a bundle out of sight (they're bundled quite neatly in the cupboard where I can see them).
Identify a socket that runs through the cupboard.
Disconnect the cable from the socket and tape on a length of new cable.
Pull the old cable back into the cupboard, dragging the new cable there.
Terminate old cable for the switch.
Connect new cable to the switch and socket