One thing though the 3socket - where does that surface mounted trunking come from/ go to because by the looks of it its single trunking indicating that the 3 socket is perhaps already a spur so you cannot run an extra socket off it but you can use a flexible extension cable.
That's a great idea about hhe extention.... might be an easier job..
Yeah it looks like the socket is spured to the side of the breast where there is a switched fuse box. It might have been for the TV for the previous occupants . I'm not sure