Well "systemctl stop kodi" does stop Kodi as it disappears from "top"!
However "X -verbose 6 > ~/xlog.txt 2>&1" produces the following in xlog.txt:-
_XSERVTransTransNoListen: unable to find transport: tcp
Failed to disable listen for tcp transport(EE)
Fatal server error:
(EE) Server is already active for display 0
If this server is no longer running, remove /tmp/.X0-lock
and start again.
(EE)
(EE)
Please consult the The X.Org Foundation support
at X.Org
for help.
(EE)
I now think that previously I used xrandr and on the problem TV it thinks these are the only supported refresh rates:-
1920x1080 24.00 23.98
LibreElec does not seem to support either cvt or gtf to create a suitable modeline!
There are a couple of web sites which create a "modeline" which give slightly different values. However in the end I copied the Intel version of the xorg.conf I created on another computer and changed the driver name to "Radeon" copied it to .config and now have 1920x1080p@50Hz!!