As I still can not send a file directly from Libreoffice to Thunderbird this is the current state of play:

libreoffice
Failed to open display
Unable to init server: Could not connect: Connection refused
/usr/lib/libreoffice/program/soffice.bin X11 error: Can't open display:
Set DISPLAY environment variable, use -display option
or check permissions of your X-Server
(See "man X" resp. "man xhost" for details)
robins@robins-desktop:~$ libreoffice --display
Failed to open display

(soffice:31790): Gtk-WARNING **: 22:49:07.874: Missing argument for --display
Unable to init server: Could not connect: Connection refused
/usr/lib/libreoffice/program/soffice.bin X11 error: Can't open display: --display
Set DISPLAY environment variable, use -display option
or check permissions of your X-Server
(See "man X" resp. "man xhost" for details)
What should I do next? This all has arisen since a clean install. Previously I could send files to email. I think I have missed out an app that should be installed, but I do not know which.
I only reinstalled the system partition. The home partition is as it was.