Thanks to those who responded to my earlier about possibly non-copper
wire in my ring main.
Now I've managed to hawl some out, I can see that it is indeed copper
inside after all, possibly coated with tin. As one poster suggested,
there are 7 strands.
The rewiring of these particular rooms by a past owner must have been at
a time when this kind of wire was used. My recent work hasn't replaced
all of it, but as part of adding sockets to my wife's study I now have
one less room with the stuff.
This is a good thing - it's surprisingly hard to wire a socket with all
those strands jumping about.