Oh FFS Facepalm.
Admittance: first time I have used nano.
Pros: learn something new every day
Cons: thought I had it all wrecked.
So, I was editing file correct, and hitting Control-O to save... and every time somehow it was saving a new file rather than saving edited file as original filename.
Crisis Adverted at the expense of my learning. As they say..."Now you know...!"
So, in Nextcloud I am down to these two errors now:
-The reverse proxy header configuration is incorrect, or you are accessing Nextcloud from a trusted proxy. If not, this is a security issue and can allow an attacker to spoof their IP address as visible to the Nextcloud. Further information can be found in the documentation.
-SQLite is currently being used as the backend database. For larger installations we recommend that you switch to a different database backend. This is particularly recommended when using the desktop client for file synchronisation. To migrate to another database use the command line tool: 'occ db:convert-type', or see the documentation ↗.
Not particularly worreid about using SWLite for backed; small operation just used for me. Maybe one day I'll figure this out but I fear converting will be more undertaking than I am ready for at moment.
I have read some stuff about first error but found nothing definitive. Im sure though, as error above, it's likely just a line commented in/out that should or should not be.