I have had a problem trying to refresh Mint Update and finally some searching has shown that apparently it was caused by some stupidity by Google. Stupidity seems to be a regular feature of both Google and Facebook these days, but luckily people manage to find ways to fix their stuff ups from time to time.
In this case the refresh was throwing up an error message:
no public key available for the following key IDs:
1397BC53640DB551.
The fix appears to be to open a terminal and run the following command:
wget -qO- https://dl.google.com/linux/linux_signing_key.pub | sudo apt-key add -
Then do a refresh.
