Ok, update. I disconnected the shifter cables at the gearbox, they run smooth, and manually selecting gears made it clear the issue is with the linkagaes/selector on the gearbox. I removed the selecting bellcrank (red arrow pointing to it below):
Once removed the gears could easily be selected on the gearbox and the in out of the selector from the gearbox springs nicely in the 3/4 plane, so the issue is in the selecting bellcrank.
The square had some wear so I rotated it 90 degrees.

I took apart the hinge bushings, greased everything up (the hinge was smooth anyway)
Anyway, as soon as I bolted the red arrowed bracket to the gearbox the in/out of the selector at the gearbox (or the left/right of the gear stick) stiffened up, so I am thinking the red arrowed bracket is the issue. Looking at it from the side it's a bit bent, downwards at the free end, so maybe that is causing it to not be aligned correctly and it binds up. The shift is a huge improvement over before but still does not centre in the 3/4 gear plane.
Does anyone have a pic of this selecting bellcrank piece so I can compare?
Cheers