The missus just took some video footage of me cleaning the windows. I don't know why. Maybe she's secretly entered me for a George Formby look-a-like competition. I wouldn't put it past her...
Anyway, it was shot with the camera (a Sony Cybershot) held sideways so the resulting video, an .mpg file, was the wrong way round.
She wanted to rotate the video and opened it in PiTiVi in the hope that it would do it for her.
But it wouldn't.
So, after a little bit of searching I found this command, which will do it for you. You will need mencoder installed... Just change the INPUT.AVI bit to the name and extension of your file.
mencoder -vf rotate=1 -o OUTPUT.AVI -oac copy -ovc lavc INPUT.AVI
Kudos to the chap who posted it originally, a Mr Jose Catre-Vandis, no less!