Hi David,
Do you have any other plugins installed? A few months ago when I first tried to use Boost, I experienced the same problem. On a new Moodle install, Boost worked. On my development site with lots of additional plugins, Boost was broken.
I started removing additional plugins, cleared caches after each uninstall, until Boost worked. Turned out to be ONE mis-typed character in the css file of an additional plugin. Embarrassingly enough, it was a plugin I maintain. MooTyper - latest version has been fixed and Boost works just fine for me, now.