Awiouy, I can confirm it works fine and sustains the settings over a reboot. I'll play with the settings for further customisation.Thank you very much!
Although the settings are retained over a reboot, if any services are enabled their .conf files in /storage/.cache/services are over written after the reboot, but not before ssh has had a chance to see which ports it should be listening on.
So first reboot you can log in on your chosen port number.
The second reboot everything is back to the defaults and you can only login on port 22, unless you updated the file after the first reboot.
Ahhhh! 
For those interested: I'm running LibreELEC (official) v7.0.2 Raspberry Pi 3 freshly installed.