Thanks to you both for your help. That fixed my problem, and now I have a new one!
As I stated above, my project settings are as follows: 720×480 widescreen 1.2 pixel aspect ratio, 29.97fps.
OK, so I do a little editing, insert a few pictures, a little video here and there, and then I decide to check the results so far to see what the rendered movie looks like. So I click on EXPORT/MOVIE, and check that the output settings match my project settings and export.
The trouble is, when I play back the rendered footage in a few 3rd party players on the PC, such as VLC or even Windows Media player, the aspect ratio is wrong. It’s as though the players don’t know they are playing a W/S video, and it plays as a standard 4:3 video.
Where am I screwing up?!