Thanks to all who replied,

That chuck was, uh m TIGHT

Its off now, got the taper cleaned up and sorted the gap piece out.
I think that chuck hadn't been off for 20 years, (could explain the
unused F/plate, catch plate and little used 4 jaw ?)
The 3 jaw sc chuck was a mile out,
I was thinking of swappng it for the 4 jaw independant I have and
investigating the 3 jaw.

Many thanks again.