This is of course an ignorant guess, but I won't be surprised if, by removing a few sockets from the ring, the location of the fault will be identified as being inside one of the backboxes, or the cable between two sockets, quite possibly where something has been screwed to the wall and the screw or a nail has pierced the cable. If so, a repair of the damage is all that will be needed.the kitchen ring main rewired.