One of the easiest ways to install extra sockets is to convert singles into doubles. However, if the spur already has two sockets, you cannot convert either of them to a double socket.
To find out whether a socket is on the ring or if it is the only one on a spur, switch off at the mains and remove the faceplate. If there are two sets of wires, the socket is either on the ring or it is the first socket of two on the spur. Attach the two tester leads of a continuity tester to the two red socket wires. The tester bulb will light if the cable is part of the ring but not if it’s on a spur. Where a socket has only one set of wires, it’s on a spur – but it could also be the last of two sockets. Call the first socket ‘A’. Look for another socket nearby that could be on the same spur. Call the second socket ‘B’. Remove B’s faceplate and twist together the black and red cores of the cables going towards A. Remove A’s faceplate and connect the leads of the continuity tester to the red and black wires. If the tester lights, A and B are on the same cable and you must not double either.
If you want a new socket in a new position it can be installed by means of a spur. You can have only one socket on this spur, but it can be a double. There are a couple of places where you can make the supply connection: to an existing socket outlet to which no spur is already connected; and at a joint box inserted in the ring circuit. If the ring runs close by and under the floor you’ll have to ‘guesstimate’ its route, then lift a floorboard or two to confirm it. Use 2.5mm sq. cable for the spur. Switch off power at the consumer unit.