rotating/flipping video the correct way

[peter maddison]

I have taken a video with my Sony Z2. I took the video in landscape mode 
(turned mobile to the left) and when I play the video it automatically 
turns the video back round by 90° so when you play it, you have to tilt 
your head to watch it.

I have installed avidemux and tried to rotate it, which works but the 
image is mirrored.
So, my next question is... How do I flip the image so it doesn’t show 
writing backwards but the right way round.

Thanks
Pete