My mother has a single socket under her kitchen worktop which she
currently has both the dishwasher and washing machine plugged into via
a 13amp multisocket. Obviously if she turns both on at once the fuse
blows in the multisocket, so she doesn't. I've just made an
exploratory investigation to see if I could change the single for a
double to make things a bit easier. I'm assuming it would be safe
enough to run two 13amp appliances from a double socket on a standard
ring main, am I right?

Anyway, she evidently doesn't have a standard ring main. Rather than
finding the two cables I was hoping for going into the socket, there is
only one. The outer sheath has been cut back so far that I can't see
it. The three cores (?) are stranded, but are substantially thicker
than the usual 2.5mm twin and earth, I would guess they are 4mm. They
are sheathed in PVC, the earth is plain green and the other two are
grey. From what my mother can remember the wiring there dates back to
the early 80s.

What I'm asking your collective wisdoms is what this is and whether I
can safely put a double socket on it which will draw 26amps
(sometimes). I'm guessing it's a radial circuit, but I await your
thoughts.

Many thanks,

Martin