The PCBs for the controller arrived today so I soldered up some of the components and tested it out. There were a few small non-critical design errors but actually it worked a treat except that I noticed that the current through the L293E's did not drop off when the stepper motor was not being used. The previous breadboard configuration was too chaotic to notice such things and I'm sure there is a simple explanation. Must dig out the L293E datasheet and look for some clues.