druiaghtagh - Try these suggestions
click here. Depends how neat the finished result needs to be. Either whack an extra bit of cable into the back of the socket [
assuming there's two sets of cable otherwise it's already a "spur" if there's on one cable]. For a "pro" job, cut into the ring as per diagram on the website. If you don't want to chase it all into the plaster etc [bury under the surface] you can get plastic trunking to cover it up.
goes without saying: [1] turn the power off & remove the correct fuse first, [2] check the socket's off [circuit tester or plug in accessory and [3] if you ain't sure what you're doing get someone in who does!! Good luck!!