If I am understanding the situation correctly, this area is behind some base carcases. I would snip the damaged cable, drag it back, make off an accessible joint in the horizontal run, run a new cable from there to the snipped cable in the vertical run (if necessary chopping out the mortar so there is more length) and fit a socket there. If there is no skirting along that wall, you could fit the accessible joint in line with the cable run to indicate its presence.