Still don't know what was going on there, but at least I'm fixed now.
Don't quite get your version numbering scheme :-) but I was running 1.2.0.98 rather than 1.2.0.0.
I'd gone through the whole process exactly as you'd expect - uninstalling it from within MC, uninstalling it by running the uninst.exe... that got rid of the one and only copy on the hard disk of playingnow.dll, along with the rest of the files, downloaded it all again from the website, but still got 1.2.0.98 back along with the 1.2.0.98 version.txt.
In the end, I actually had to open up the mjp, get the link from inside it and manually download the ZIP file and run pn.exe from inside it!
So somewhere something was caching an older version. No idea where - it can only really have been Media Center doing it though!