It’s sorted!
I went into the Club workshop yesterday afternoon complete with wedges to
have another go.  There was the chuck neatly sitting on its side on the
drilling table complete with a long screw and the metal block from out of
the `slot’.
Immediately behind me came a bunch of members laughing their heads off. 
It had taken one of our more junior members to quietly work it out on his
own.
Someone had been there in the past and made a `repair’.  The chuck had
been drilled and a long hex-headed screw placed through and up into the
aforementioned cross-slot.  The steel block in the slot was a `nut’.  I’d
guess that the chuck had, in the past, developed a tendency to come off
the taper.  The screw was fitted to keep it in place.  The taper (it is
JT6) does have a bit of a battered look!
Without the screw and block in place it’s obvious that the slot is there
to prevent turning whilst one winds the knurled collar down to remove the
chuck.  I was fooled by this addition but so were three or four other
old-farts!  It’s good to know that there is some good sense and lateral
thinking in the young ‘uns.  Thank you Chris!