In uk.d-i-y, Martin Angove wrote:

Depends on the cable run though. If the cables are under the floor or
above the ceiling (very common) there may not be a suitable place to
mount a socket. Come to think of it, apart from putting the extra socket
bang next to an existing one (in which case, why not spur off that) I
can't think of the sort of cable run which would allow you (easily) to
insert a socket in the ring unless it is entirely surface-run. If this
was possible in the OP's case I don't think he'd have bothered asking
the OQ!

Not that rare, actually: the point is, once you've opened up the ring
at Socket A by disconnecting the cable which runs on to Socket B, you
have the full length (in principle!) of the run between A&B to play with,
making it likely enough that you can find somewhere to put the new
on-the-ring socket using all or most of the cable which previously
ran A-B. Then you put in two new cable runs, either from new-to-A to
reinstate the ring with a spur off to the-new-socket-you-actually-wanted,
or incorporate the new-socket-you-actually-wanted into the ring by
running the first new cable from the may-as-well-be-a-socket-as-a-juncbox
to the new-socket-you-actually-wanted, and the second on from the
new-socket-you-actually-wanted on to A.

(Next week: Stefek explains "one song to the tune of another" in no less
than 2,000 words ;-)

Cheers, Stefek