I solved my own issue but surprised that it hasn’t been reported (or I missed it when searching), so posting here for posterity.
When I first installed OSMC on my RPi2 (way-back alpha-4) I set-up a separate SSH account as I wanted to use the Pi also as a remote file server via public-private keys. Everything was working fine until a recent update, when I suddenly could not access over ssh. Not sure which update, as I haven’t logged-in an a while.
Anyway it caused some consternation because I was working remotely at the time and couldn’t access my files.
When I got home I found that my ssh setting had been reset to default, and I was getting uid errors when using sudo (/etc and /proc were now owned by OSMC, not root).
In short, I solved it by editing the sshd config file directly on the RPi, which had also now allowed root login again, and had removed me from ‘AllowedUsers’.
sudo nano /etc/ssh/sshd_config