Ok I've cracked it for sure thanks to http://www.macosxhints.com/article.php?story=20070424081346722.
You have to create the library on the Mac FIRST, then add the .itl extention. It means reimporting the music again, which I guess means loss of play counts and playlists, but after you've done...
That's exactly the problem I have right now, I found this forum Googling with my problem and signed up just to add my support.
It seems to me that Windows and OSX iTunes use different formats for their libraries, making them incompatible. If that is so, which is a ridiculous thing to do...