"A big mistake not getting that socket moved before the units went in."
Yep, the reason why I held my hands up.
I suppose all is not lost, the internal cable comes down a wall in our dining room and then along the skirting board at the rear of the cabinets. There possibly may be enough slack to cut the wiring and reterminate in a new "modern" master socket.
If not I would at some stage bite the bullet and move the cabinets out to arrange for a more satfactory placement of a new socket.
Installing another Master socket on an extention from the existing one is just asking for trouble.
I was referring that the cable to the existing master socket could be cut and reterminated in a new master socket in a more convenient place than the current location behind the cabinets.