Duncan,
As you have experienced video's always play on top. That is standard
behaviour in PowerPoint.
But maybe this will help you:
http://officeone.mvps.org/powershow/tips_text_over_video.html

Signature
Luc Sanders
(MVP - PowerPoint)
> How do I insert animated text onto (overlayed) a movie in a slide so that
> when the movie plays during the presentation the text does not disappear -
> I
> have tried ordering and grouping but it does not work.