So, I figured it out. It seems some files to do with Melodyne Essential that came bundled with my audio interface wasn't fully uninstalled. They were in a folder in a folder called ReWire (library/application support/propellerhead software/rewire). I moved that to my desktop and now it works...
Thanks, but as I've said, I've repaired disc permissions and deleted preference files. When I repair the disc permissions now, nothing really happens; and I can find neither of those files when I search. There's nothing I can relate to garageband in the preference folder.
I found and deleted...
Whenever I try to open Garageband, the box with 'initializing' appears for a few seconds before an error saying 'Garageband quit unexpectedly'. I've tried running the disc utility and repairing disc permissions, I've tried deleting preference files, I've re-installed Garageband from the disc...