Thank you all for your very good comments.

Rick - I will redesign the frame and go with 2 ball screws as suggested, possibly 16mm 10pitch.

Jonathan – In my initial sketches the axis was constructed in a similar manor to your drawings but I found the spindle did not cover the whole of the table. So I either extended the frame of the base or took the rails to the extreme of the axis allowing the spindle to go past the work surface while keeping the foot print of the machine as small as possible. But as you have pointed out it has created a weakness and I will take this on board.

JAZZCNC – I also agree Jonathans suggestion is a good one, but as for the protection of the ball screw I was hoping to use brush seals fixed to the gantry to prevent wood chips causing damage. I’m sure I’ve seen this being used on a machine on this site.
I also like your design for the gantry, so I’m back off to the drawing board. I’ll keep you posted.

What are your thoughts on these Chinese screws as I don’t want to work to microns?

As you are all well aware of, everything is a compromise.
Jim