I finally got round to applying the electrodes and waking the beast from it's slumber (I plugged in my RepRap).
It's been stuck behind the TV for a while until I had a free evening to start configuring and testing everything. I spent an hour sorting out an Arduino communication issue (I'd foolishly uploaded the firmware compiled in test mode, and it kept saying 'start' down the comms channel).
Using ReplicatorG, I was able to jog the x-axis a little back and then forth - which worked!
I then wiggled the y-axis a bit - and that worked too! Then Z - both up and down worked!
With my hand on the PSU power switch, I gingerly pressed the 'home' button for the x-axis. The x-axis moved back to the home position, triggering the endstop and stopping on it's own! Then the same for Y - and Z - Things are going really well!
I spend a few more minutes winding the x, y and z back and forth. All was well until a z-home caused the wobbly z-flag to hit the side of the endstop, meaning the bed tried to ascend to the attic, winding itself into the print head on the way. The z-axis on the motor slipped, meaning that the bed was badly skewed before I got to the power switch. The z-belt also fell off.
I put it back together again - I don't think it's damaged but it gave me a scare.
Note to self: Hand on panic button ( the PSU power switch) until I'm really confident it's working.