If you go to
http://www.theiet.org/publishing/wiring ... s/updates/ there is a PDF called BS 7671:2008, Corrigendum (July 2008) if you down load at the end there is a diagram which shows exactly what you are allowed to do.
You can either convert single sockets by removing back box and replacing with a double box and the new socket will sit like old one did. Or you can get special sockets which allow a double socket to be fitted in single socket box these stick out a little more than normal socket.
Where you already have a spur you can't add a second and from what you are saying you already have spurs. You can swap for double socket.
When adding spurs first you have to locate a ring main this will have two cables and when tested will confirm the ring is intact sometimes people in the past may have not followed the rules so just because there are two cables don't automatically assume it is a ring.
If you can't test you should not be doing the job. Loads in projects section and also be aware of Part P and the new regulations which require all sockets now to be RCD protected.