Once I'd plugged in the installed steppers into the driver circuits, initial testing showed that the heatsinks got *very* hot. After a couple of minutes, the x-axis started to 'jiggle' - oscillate about half-a step backwards and forwards making a juddering noise. I switched it off.
I moved each potentiometer to minimum, and the steppers stopped moving completely.
Using the stepper exerciser and the 'home' command, I set the axis moving. Turning the current up slowly, you reach a point where the axis just starts to move. I gave it 10% above that and then stopped. That gave it enough to move all the axes reliably.
The heatsinks still got *very* hot, so I took off the standard ones, and grabbed a few from my spares box. Due to a few years of rebuilding PCs, I had several PC heatsinks around (for those who are interested, they are a from a 486dx2, a Pentuim 75, and an Athlon 800). I drilled a 5mm hole near the edge and slapped them on. Now, even after a while, they only seem to get warm.
I also extended the stepper plugs using some 4-core ribbon speaker cable.