I found a suggestion on another forum which seems to partly solve the problem:
https://askubuntu.com/questions/851810/unable-to-install-snap-package
So now if I run :
sudo systemctl restart snapd.servcice
And then
snap refresh
The snap refresh works ok.
However it is not a permanent fix as if I wanted to install a new snap I need to run
sudo systemctl restart snapd.servcice
again before each new install/update.
Again if I leave my unit overnight the automatic update seems to fix the issue.