What is the contents of ~/Library/Application Support/TextMate ?
Well, there was a copy of 'TextMate.tmbundle' in 'Pristine Copy/Bundles', so I moved it to Bundles and restarted TextMate.
It worked! My 'Language Definition' bundle disappeared, and there was a TextMate bundle in my list instead.
I think that's fixed it. Thanks everybody for the advice. :-)
And incidentally, I noticed the Themes folder, and realised that my syntax colouring was independent of my preferences (probably obvious to most people).
Bruno