When I try to play portrait orientated videos (I know, I should stop recording in portrait) they play back in landscape orientation so they are ludicrously small.

I have tried to lock the orientation but it doesn't work. When the video is previewed in Gallery it's the correct way round but as soon as I tap the play button the screen reorientates.

How do I stop it doing this?

 

While I would still be interested in the answer to this, I have sidestepped the problem by installing MX Player and setting that as the default video player. MX Player doesn't have this annoying problem.