Joining/mending the cable is one aspect, and hiding it neatly is another. Client put up a shelf unit, and one of the fixing screws punctured a cable to an adjacent switch which controlled the overhead light. She didn't know about zones, and the screw was directly vertically above the switch. It was a dot n dab plasterboard wall, so not much room to work with. I dug out the plasterboard and sank a 16mm back-box, cut and jointed the cable, put a blank faceplate on and moved the shelf a couple of inches so the faceplate was obscured by items on the shelf, and stuck a label on that said "electrical joint behind here". A picture could have been hung over the faceplate as it was at a suitable height. The joint is safe, it's accessible, but virtually invisible.