OK, so I’m a dumbass

Well manual tagging works OK, and the Category gets picked up.

So what was missing? Well I went through the installation of the tag plugin again which was when I realised that I’d copied a default string with a dummy path into my mt.cfg file instead of one with the real path.

Doh!

So now I have at least the lovely tag interface working. It still remains to be seen whether these things get picked up by technorati.