Our house was built around the early to mid 60s (so has "modern" PVC twin &
earth cabling) and has just one ring main feeding the whole house. We want
to add in some extra sockets but obviously I don't want to overload the one
circuit so I've had an idea that I want to run past you peeps here.

Most family/friends houses seem to be wired with an upstairs ring and a
downstairs ring. My idea is to go to all the sockets in the upstairs rooms
and basically pull the cables out of the sockets, back down to under the
floorboards and joint them there (either with "traditional" junction boxes
or with crimps/heatshrink sleeve), so that the continuity of the ring is
preserved but it is now just serving the downstairs sockets, then run a new
ring for the upstairs sockets, and adding new sockets to each ring where
needed.

Is this OK?

TIA,
Bill