Problems Upgrading Mosaico 2.0 to 2.1
I had a closer look at all the folders and saw there was a symlink in the templates folder pointing to the templates folder! This must have been preventing the upgrade. I think the symlink was there because earlier we had had problems with mailings not finding the social icons and had created a symlink to solve that problem and then forgotten about it!

Once I removed the symlink Mosaico upgraded fine.