Thanks for the quick reply! I eventually uninstalled the driver AND compat5x and reinstalled both. That got it working.
Then I started making a new xorg.conf from scratch. I managed to get Composite with RenderAccel working, and translucency/shadows in KDE. Unfortunately, it only works with one monitor - when I turned Xinerama on and configured my second display it all falls over. I can log in, but as soon as I get to the desktop, my icons disappear and start flickering on and off (more off than on), the dock is barely visible and I'm left with a black square from the KDE splash screen.
I don't know if it's any use, but I'll attach my dmesg and Xorg.0.log. If there's anything else that is useful let me know.
Spose I should put the obligatory thanks for the work on the driver comment in here
Not many companies make an effort to support FreeBSD. It's appreciated.