No worries Eddy, and thanks everybody especially John. The circuit looks easy enough, I should be able to test it sometimes next week.

Alternatively, I have found an off the shelf converter from Axiomatic:

http://www.axiomatic.com/TDAX13020X.pdf

But I have a feeling it's going to be much more expensive than the diy version John proposed, so not really an option.

On another note, can somebody confirm a 24-28v 10A power supply would do, and what voltage do you feed to stepper drivers?