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Double sockets are still only rated at 13A not 26A.
You can quite safely add another socket, just check the wiring in the one you want to extend. If there are two sets of wires then extend the ring to a new socket.
If there is only a single set of wires then it's a spur. You can extend a spur but you'll need to add a 13A fused connection unit before the socket you are spurring from.
Cheers.
The fuse box is in an adjacent brick outhouse. Idk for sure whether it starts there or is feed off the main outhouse room, but the wires come out of the outhouse, under the ground, into the shed and through the conduct.
I like the idea of another socket, but given that there are a total of 4 sockets (2 x2), and idk how it is supplied / feed(?) just moving that one seems safer.
NB. in the last pic: left = double socket , right = double socket , up = light and light switch