Thanks, Jeroen.
Well on that basis, all the plugs I have here are indeed 50 ohms, then.
In fact I pulled a centre pin out of a new packet from the same batch
and it fits the sockets in the VNA perfectly. They all accept the pin
with a nice snug fit and what's more the sockets show no signs of ever
sustaining any damage/spreading/flairing/bending and that goes for
those on the T/R bridge, too. On reconnecting everything, however, the
problem still persists, although the 50 ohm load is now showing as very
close to a dead short; virtually the same spot on the Smith chart
overlay as when the socket is shorted, in fact. Once again, the other
loads do likewise and they all check out fine as 50 ohms with a DVM.
Very strange! I wonder if there's some setting I may have got wrong
somewhere. :-/