MP4 playback - aspect ratio?


I have a bunch of MP4 videos which my HDR-Fox will happily play, but not in the correct aspect ratio - old 4:3 ones are stretched to fill the screen, as are files wider than 16:9. They play correctly (with black bars at the sides or top/bottom respectively) when served by the Humax to a Roku box over DLNA. One has a subtitle track and this is also not selectable on the Humax using the "sub" button (also not visible on the Roku btw..). Live or recorded content will play happily on the Humax in the corret ratio.

Is there any way for the Humax to honour aspect ratio settings on MP4 files??
Many thanks... Will give the ideas in the link you've posted a try (sorry for not checking there first...)