I think the significance of not using different materials is that aluminium has a much higher coefficient of expansion than steel - this could be important as my workshop temperature has varied from 4 degrees to 30+ degrees. as far as the order of accuracy that I want, the magical 1 thou will do. I have got an old optical workbench (just a 1000mm by 600mm by 200 mm 'T' slotted extrusion, heavy enough to need two people to lift it) that may as well be the sub base.