Having found that FF & rewind was working I decided to persevere with LibreELEC-RPi4.arm-11.0-nightly-20220218-29c7284.img so had a few more goes.
I eventually found that setting the HDMI port from 1 to 3 and the physical port from 3000 to 0 partially worked. I say partially because only some of the buttons work, others (STOP, FF, Rewind, PAUSE) do not.
My current assumption is that I need to edit remote.xml keymap. If so would it be possible to copy one from my RPi3? If not is there a simple guide anywhere? I'm reading through https://kodi.wiki/view/CEC and following links but its certainly not what I'd call simple
At least I'm making some sort of progress