What I'm after really is an idea of the size of wiring I need to use to
extend the ring (I'm planning on just breaking into it at an existing
socket, and then taking one wire to a new socket, and then joining the
two
sockets together to rejoin the ring).



A spur for a double ganged socket outlet should be wired from a ring main in 4 mm (or
bigger) cable
or, alternatively, continue the ring through the new socket, IMHO.

Professionals? - comment please ?

Nick