Thanks, I'd read a bit about pixel width as opposed to dimension changes, and am (justifiably?) confused.

I think the problem may lie with the output from Sony Vegas. When viewing the thumbnail of the video in Windows it is 4:3, and looks a different shape to some 16:9 video thumbnails I have. Also, when played in anything other than Windows Media Player the video is not displayed as widescreen, but stretched to fill my whole 1024x768 monitor. This happens with VLc media player for example.

This leads me to think that maybe Vegas is not exporting it as true widescreen, despite selecting Pal Widescreen AVI as my export format - but this may be linked to the common 720x576 resolution as Alan mentioned.

Can anyone shed some light on this?
Thanks