Why didn't I think of it before?

If I replace the 70T spindle drive belt with a 60T one the motor moves forwar by 25.5 mm giving enough room behind the motor for a much neater arrangement for driving the Z axis. I have to scrap the plastic belt cover and make a new motor mounting plate supported on pillars but that's no problem.

Revised Z layout CAD file here ..still provisional of course. I've also changed the stepper motor drive belt from 65 to 75T to enable the motor to drop down behind the column.

Russell.