'Enable access for assistive devices' not being set

I've done a clean install of Mountain Lion, but when I tick the 'Enable access for assistive devices' box in the accessibility pane, nothing happens.

I've trashed the 'Accessibility' preferences in the user 'Library/Preferences' folder, and when I choose the 'Mouse & Trackpad' option, the plist was added to with the relevant Dictionary, but nothing is added when I tick the 'Enable' box.

I've got a backup, so if possible I could replace the preference pane setting app, if I knew where to find it.

Is there any advice anyone can give me please, or should I do a re-install ober the top of my current install?

Regards

Santa