Rotating movie taken on iPhone ?

Any ideas how to rotate the movie please.

What you can or should do depends on your specific situation, goal, and/or app/system you plan to use.

Background Info:

1. Your iPhone embeds the correct playback orientation information within the file.
2. If you play the file in QT-based apps, it should play correctly in the original portrait orientation as is.
3. If you play the file in most third-party apps, the orientation data will be ignored and the file will automatically play in a landscape (sideways) orientation.
4. Any QT based app that plays the file in the correct orientation and has "Export" capabilities can re-encode the file in the correct orientation by simply exporting the file data to a new file. (Be advised that apps like QT X may need to "force" an export—otherwise it will simply perform a "Save As..." operation—while apps like QT 7 Pro have separate Export and Save As... features.)
5. Any conversion app that ignores the embedded orientation data and has "rotate" and export features can rotate and export the file in its correct orientation.

Since you have not indicated what apps/systems may be available to you, I would normally recommend an app like MPEG Streamclip which is both free and available for use by Mac and Windows users.

Any tips would be welcome, as I dot want to start downloading new software just for one movie. ??

Not a Facebook user so the only way to be sure the file plays correctly in all situations would be to re-encode the file data with the correct orientation. If you have QT 7 Pro installed on your system, then then open the file in this app, check to ensure it is playing in the correct orientation, and then export the file to settings compatible with Facebook. If the file is not playing in the correct orientation in QT 7 Pro, then change the video track visual setting to the correct orientation before export. If you don't already have QT 7 Pro installed, then use whatever converter you have installed on your system to correctly orient playback before performing the export operation. I mentioned MPEG Streamclip as one of several possible alternatives here since I'm not a Windows user and not overly familiar with what choices may be available to you. The key here is that you must both correctly orient your playing file and re-encode the data to ensure the final file plays correctly in any media player.