Thanks for your posts, helped put me on the correct path here.
For the record, using Devuan jessie 1.0.0 LTS (systemd-free Debian jessie fork) the necessary commands before using rclone were:
eval `ssh-agent`
ssh-add ~/.ssh/id_rsa
That, of course, after the ‘traditional’ (as in, ‘not even sftp will work without it’) steps of creating and transferring the public/private key pair on the machine and user running the rclone client, and transferring the public key on the rclone SSH remote.