Sorry everyone. This turned out to be an issue with the stupid Synology NAS, not with Duplicati.
There’s a bug of some sort in the Synology DSM software that means connections on port 22 simply don’t work, even if they’re enabled and configured correct.
I had to change the SSH and SFTP ports to a different port number. Duplicati could then connect.
Apologies for wasting everyone’s time! 