Hiya Jon,
I'd put a good steering lock on, and get an alarm fitted. You can get a Scorpion Cat 1 alarm / immobilser fitted for ?300. There's an official Scorpion fitter in Dundee.
I reckon it was targeted by some local t*ats who saw it being worked on, maybe saw him driving it that day, and thought that'd be great to f**k about with.