Instructions: How to Build Plex Media Player on Raspberry Pi 4 B Under Raspbian

Hi All
I still for the life of me cannot get HDMI-CEC working on this. The PMP on the Pi4 otherwise works great, but I am stuck using a keyboard on my TV which I just don’t want.
I booted up my old Pi3 running Rasplex, and when Plex pops up on that, down the bottom right corner, is a popup box which says “Pulse-Eight CEC Adaptor” and then says “Connected: TV”. My TV remote up/down/left/right/select etc works perfectly, so you can navigate the Rasplex menu with the TV Remove.
But for this installation on the Pi4, it doesnt work.
I have tried to manually install the Pulse-Eight from here: https://github.com/Pulse-Eight/libcec/blob/master/docs/README.raspberrypi.md and that built/installed fine, but I dont know how to now get the PMP to use that for CEC.

My TV is a Song Bravia 40" or something from about 10 years ago, and as I say it works perfectly on Rasplex out of the box. No idea what Pulse-Eight really is, but it seems to be preinstalled on the Rasplex and loads up the HDMI-CEC on startup and just works.

Does anyone have any idea?