hi Peter, ah things are a little clearer now,
obviously something has gone amiss with either plug or socket, and got twisted preventing the two mating up, I would suggest looking at both starting with the socket.
if it is correctly aligned looking at it front on there should be 3 fixed lugs one at 20 to one at 12o clock and one at 20past.
in the centre of the socket where the pins holes are there should be a square locating groove at half past, if this is not alligned the plug will not connect,
however if this is correct look at the plug this should be a miiror image of the socket exept instead of a fixed pins there will be grooves on the outer thread and a square peg at half past that locates in the groove of the socket, if anything is out of line it should be obvious, and will require the component to be stripped down to rectify the problem,
one other point on the plug make sure the the outer screw is fully unwound anti clockwise as even just a bit will stop the plug locating in the socket ,
if you require more help just say so I will post a couple of photos of the correct alignment.
colin