Folks, this actually works, unlike the plethora of "official" methods which were suggested to me,
all of which failed to work.Survives reboots, and is a GUI and doesn't need messing around with rc.local, crontab editing etc:
https://www.linuxuprising.com/2020/12/remap-keyboard-and-mouse-buttons-on.htmlhttps://github.com/sezanzeb/key-mapper/releasesMy K780, from Logitech, was designed so that Function keys' primary roles are MEDIA CONTROL - as to why this daft decision was made, who knows.
This WORKS! I don't care what the "official" methods are/were for starting services on boot, I am sticking with this as it functions correctly. Tested on a Logitech K780 kb connected via BT to a Raspberry Pi 400 running the latest "Buster" version of Raspberry Pi OS