Do it yourself and it will cost you £20.
Get a pro to do it and it is a question of how long is a piece of string! How long will the spur be? how easy is it to get access to the ring main to spur off? How neatly does it need to be done (will the cables be buried or surface mount etc etc) call 3 and get some free quotes done porbably looking at £100 minimum as it seems impossible to get anybody to do anything for less these days.