Ok, to get that chuck off I used bottom gear and chocked the chuck,
tight, to the bed,

I also used some plus gas penetrating stuff,
I'll aslo admit to using a 2'6" tube over the c spanner (over half of
this was on the spanner)

The problem was rust,just on the last thread of the locking ring. The
taper and key are fine so I'll strip and service the 3 jaw and try it
again.

While the chuck was off I sorted the gap piece out as well.

not yet done the 3 jaw as I'm busy with work and I've still to build
the rotary phase converter to run this lathe and my Elliot 10" shaper.

Lee