Thanks for the info guys.
The power supply build looks a lot less scary than I expected, and I will defiantly consider this when I get around to buying electronics.

Regarding drivers I plan on just using 1 motor to drive both Y axis screws, so will go with the cheaper drivers.
Does anyone have any thoughts on how well a single 3Nm motor will move a 40Kg gantry, assuming im using 1610 screws and ~70V PSU?

Other than noise, are there any other major benefits of digital drivers over analogue drivers?

At the moment I am finalising the mechanical side of the design, particularly the stepper motor brackets and connection to the ball screw.
The plan is to use 16mm HTD belts on all axis.
To connect the two Y axis ball screws I will go with the setup jazzcnc sugests here
http://www.mycncuk.com/threads/4513-3-Axis-CNC-router

I just need to get a custom end machining on the ball screws so I can fit 2 pulleys on the drive end. All 3 X and Y axis screws are the same length, so will just get the same extra long end machining on all for simplicity.