Found it!
created /storage/.config/autostart.sh
(accessible through samba share /configfiles/autostart.sh)
from terminal set it as executable (chmod a+x /storage/.config/autostart.sh)
then put this in it: (if editing from Windows don't use windows notepad - use notepad++ or it messes up the line ends)
#!/bin/bash
(
sleep 5
echo -e "connect xx:xx:xx:xx:xx:xx\nexit" | bluetoothctl
sleep 5
echo -e "connect xx:xx:xx:xx:xx:xx\nexit" | bluetoothctl
sleep 10
echo -e "connect xx:xx:xx:xx:xx:xx\nexit" | bluetoothctl
sleep 10
echo -e "connect xx:xx:xx:xx:xx:xx\nexit" | bluetoothctl
sleep 10
echo -e "connect xx:xx:xx:xx:xx:xx\nexit" | bluetoothctl
) &
replace xx:xx:xx:xx:xx:xx with the mac address of your bluetooth speakers (you can see it in the Bluetooth screen in Libreelec config)
I put it in lots of times as it was sometimes failing in testing (possibly because I was too far for the BT speakers). Doesn't seem to adversely affect it so I left it.
Putting the commands between ( ) & means they run whilst Kodi is starting, rather than Kodi waiting until done.
This works on RPi2 with BT dongle and RPi3.
Hope this helps.