Thanks.
I'd wanted something to quickly view them - and perhaps do rough edits.
I'll happily plead ignorance, but couldn't get iMovie or Final Cut to play the files. Perhaps on import they'll work, but I just wanted a quick click-to-play solution.
Premiere will open them, but I don't have the necessary to learn it and so it seems overkill for what's required but useful for detailed edits.
VLC has jerky playback on my machine, but I've found that Toast VideoPlayer plays ok - so that's a start. I still want to find an editor which is simpler than Premiere.
I didn't want to be re-encoding just to handle them, so hadn't thought about Handbrake.
Those links look to have the solution - thanks.