Bizare! Check all the earths and the ignition switch, perhaps it keeps clicking over the the 'start' position or something.
I'd trace all the wiring from the starter as far back as you can, look for damage to the wiring or connectors. Also check the wiring on the motor itself, I had a similar problem with the starter on an AX, turned out the plastic 'lug' between the terminals had snapped and so when you tightened the live connection the wire twisted round to touch the other terminal, so the motor would run constantly when the battery was connected back up.
Take the dash under tray out and check all the wiring behind there, and take the fusebox off its mounts so you can drop it down and look at all the plugs in the back.
