Thanks Andy and Akio.pkg_delete -r '*' works well, although there were a couple of packages it couldn't delete (they scrolled off the top of the screen too quickly for me to make a note of them) I was able to install kdebase4 and the other things I needed.
Well, this begs the obvious question, did you remove or try to remove the packages using pkg_delete?
Yes, I spent quite a long time using pkg_delete, working my way through all the dependencies before I installed the newer version of KDE. It took so long and I still ended up with a broken KDE that I figured it'd be easier to blitz all the packages and start again.
Ray