Well the 5V supply for the parallel interface didn't fix all the problems. After a bit of cable swapping it turns out that one of the motor driver boards is faulty.
I can get a spare from Hong Kong for about $12 but it's $45 for postage I've got a few gecko G250 boards that are currently unused so I'm going to substitute one of those instead, I just need to make up a PCB to mount it on. [I do have a Gecko G540 and I'm tempted to give it a try if it will get me cutting sooner!]
Mechanically, all axis move smoothly enough but I've know way of telling if the ball-nuts have any/enough grease in them. There's no grease nipples supplied and the construction will mean I'll have to almost completely disassemble each axis to grease it. At least the rails are easy to lubricate.
I still need to source a suitable aquarium pump for the spindle as the supplied non-submersible pump doesn't look as if it will last long.