The four holes to mount the stepper have now been drilled in the square end plate, and the stepper and mounting have been successfully mated up to the rotary table.
I decided to use M5 nuts and bolts to attach the stepper, as I've never been too comfortable with the idea of threads tapped into aluminium. So the holes ended up being 5.1mm diameter.
Here is a photograph of the stepper motor and mounting connected to the table:-
I ran up the stepper and shifted the rotary table round under power; the system worked perfectly.
Here is a photograph looking along the inside of the mounting, showing two of the table attachment screws, as well as the coupling:-
That is the mechanical portion of this tool build completed, now it is over to the electronics - ruggedising the motor leads and fitting all the PCB's into a die cast box.