Sorted (after a bit of trial and error).
(1) Downloaded KSZAQ's S905 build
(2) burnt it using LibreElec USB-SD creator
(3) Loaded it up off USB
(4) SSH to device and ran installtointernal
And now I've a fully working bootable LibreElec on my Beelink Mini MX
Pushed my luck a bit and tried to install Kodi 17 upgrade but that bombed and I had to redo the above.
Now my observations:
Some USB sticks work and some don't - I ended up using and Micro SD Card in a USB adaptor. I proved this by trying several bruning them the same way as above.
Also need to be careful which USB port (this has been stated on another post).
So off I go. Pretty quick boot up, shame I can't put 17 on though as I love that new skin.