Hahaha! Yes, I sometimes feel deranged to the point of no return, in a happy-mad way that is.
This works for me only once, then I have to re-engage the image in the filmstrip with a mouse click.
Curiously, the F3/4 keys work as described when used with the Fn held. So the keyboard must be (unlike previous keyboards I’ve used), continjually in ‘media mode’.
Thanks for your help.