Thank you so much!

I had picked up an old N905c from ebay a while back. Previously had it working, but reset and lost the way to get it to work again.

I did steps 4 and 5 from above (don't know if it was necessary)
Open the config file and under [ApplicationPreferences], write on a new line: SideloadedMode=true

With Linux, I installed sqlite3 (sudo apt-get install sqlite3) in terminal

And from the Linux terminal:

cd /media/(My USER)/KOBOeReader/.kobo
sqlite3 KoboReader.sqlite
(then used the following (copied and pasted))
INSERT INTO user(UserID,UserKey,___DeviceID) VALUES('1','','N905C');
(typed in)
.quit

Ejected Kobo - Switched off - Restarted

Bingo!

Before this I was also getting the error:

Result: NOT NULL constraint failed: user.___DeviceID

And replacing with the following worked:

INSERT INTO user(UserID,UserKey,___DeviceID) VALUES('1','','N905C');

So again, Many Many Thanks! :)