In reply to G4OWG:
About the time of the release of the first ND’s there was a lot of
stuff on the '817 groups about power leads leaving part of the
insulator (yellow) in the socket.
That turned out to be my problem. In my case the broken piece was black, making it hard to see; tell-tale sign is that it will rotate inside the socket.
I have just managed to get it out (in pieces) using a jeweller’s screwdriver, a small amount of force, and a lot of patience. Other plugs now fit.
Many thanks for this information.