As the battery cover has vanished off my girlfriend's bike. I thought that if anyone has one of these lights damaged but with an intact battery cover, that could work. Unlikely I know. Oh well.
I've posted this in the main (rather than "wanted") section of the forum, since people who only have stuff that they think nobody would want (like a broken bike light) might still read it here. Hopefully nobody will be too offended.
Progress: I've emailed Zyro, the UK distributor, and they've replied to clarify that it's the front light that I need the cover for! Wonder where this will lead...
Happy ending! She found the cover in the car park at work.
Meanwhile, the Cat Eye distributor customer services people had emailed back clarifying that it was the cover off the front light that I was looking for so that they could "assist you further". A good argument against el cheapo accessories (and specifically in favour of Cat Eye).
(and they're not paying me to say this either, honest)