I should have noted this at the time, but I was not aware of it. It was only when I tried to do the same thing with an SPList object that I ran into trouble.
This is why - BeforeProperties and AfterProperties do not work on a list
So I ended up having to create a Just Updated flag for the user to click when they want to send an email. Just before ending the event handler, it sets that flag back to false. Yet another hack to make up for SharePoint 2007's "by design behaviour" (sigh)
So if using the code in the previous post - document library only, lads. Sorry.