Fiddled with it now. It is part of the socket. It surrounds the tubes. It is a friction fit as are the tubes. Took the front plate off and could see how it fitted. Pushed in in pushed the tubes back that had gone in, put it back together to find a tube on the table. Turned out to be tube 12 which is something to do with a trailer sensor. Message- check your socket to make sure all is tight and that the tubes are still in place.