This is not a question!! I wanted to post my experience of copying instances that include Portals solutions with you all as I'm sure I'm not going to be the only one to experience this.
After completing an upgrade to the latest Portals version from a v7 adx installation I copied our fully functioning production instance of CRM with Portals installed back to a sandbox. The sandbox already had Portals installed as it had been used to develop the upgrade.
The issue on the sandbox portal after the instance copy was that I was getting a 'generic 401' error when trying to load the portal. It looked like an authentication issue but I couldn't be sure.
After confirming that the website bindings were updated and correct (didn't fix the issue) I went in to the portal administration for a look around. Again, I confirmed that the bindings were correct here.
The next step I took was to confirm the dynamics 365 instance. It looked OK but as a test decided to use the 'Manage Dynamics 365 Instance' option. Rather than switch to another instance of CRM I simply used the details that should have been preset for the sandbox and clicked update.
After several minutes I retried the portal and it worked!!!
Hopefully this information helps someone in the same situation!