Y-axis rear plate...partly to help keep crap out of the leadscrew etc Thought it would add some extra sideways/diagonal rigidity...can easily drop it.

Z motor - probably didn't think it through quite correctly...with that arrangement, the y axis front plate is considerable shorter than it would need to be to have the leadscrew & motor attached.
The rail lengths are pretty much spot on compared to the screw length - maybe i've got the Z axis linear bearings spaced further apart than needed? (170mm outer face to outer face along the axis). I'll look at swapping the leadscrew & motor to the Y front plate.

Base frame - do you mean raise the lower shelf/surface, or reduce the overall height?
I don't really want to lose much overall height if possible as it will be at a good height for me to work with it at...rather than being low down on the floor like my last mdf machine which was a major pain in the back.
I don't necessarily need all that adjustment - I could raise the lower surface & add more cross bracing to the outer frame?