Thanks Léonie, but my settings are exactly as per your screenshot, and I've tried all other options in the RAW+JPEG menu, without success.
I'm sure the movies are not corrupted as they play in QuickTime Player and can be drag-imported to iPhoto with no problem at all. (They cannot, however, be dragged from iPhoto into Apeture, as still images can.)
I have just tried importing a couple of different movies but the same problem occurs. It seems as though some component of Aperture is missing. Is that possible? Should I reinstall Aperture?
Have you run any cleaner applications, that clean caches? If the file
com.apple.xpchelper.cacheis missing from the directory /System/Library/Caches/
then videos cannot be imported or played. Have a look, if this file is still there. If not, reinstalling Mt. Lion should restore the file; or download it from the post below.
Fantastic! Problem solved, thankyou so much Léonie.
Yes, I do sometimes run Mt.Lion Cache Cleaner when problems arise, and guess I must have done so on this OS installation.
I didn't need to reinstall Mt.Lion in total -- I decided to see if running the 10.8.2 update would work. It did. And yes, I have made a copy of the cache file in case it's needed in future.