Neil,
I was helping a friend migrate a Blogger blog (which uses Atom) to WordPress. I couldn’t use the Blogger importer, as I couldn’t reach her for the account password, so I found myself in the same position as you. Here is what I did, and it worked for me:
1) You need to find an XSLT processor. If you’re on a Mac, you’re in luck: download TestXSLT from entropy.ch at http://www.entropy.ch/software/macosx/welcome.html. If you’re on a PC, you should be able to find something similar.
2) Download this Atom to RSS XSL template: http://atom.geekhood.net/atom2rss.xsl
3) Use your Atom file as the source XML file.
4) Use atom2rss.xsl as your style sheet.
5) Process. The result is an RSS file that you can import into WordPress.
