dsc810 wrote
@Quentinc
The 'flash' is believed to be a bird much closer to the camera so out of focus passing across the viewpoint.
I saw an analysis of it as a 'still' somewhere on these forums showing it was not aligned with the path of the aircraft.
It was probably on another valuable post which has since been deleted.
Edit Siloesid made this comment redundant
There is an out of focus object in the following video. It appears to cross the path of the Hunter but in reality it will be much closer to the camera. Out of focus pollen, insects and birds along with bright lighting conditions create this illusion.
See from 0:22
https://www.youtube.com/watch?v=pvHplYmh2f8