Hi DiggerDavey,
I'm Sumit, here to answer your query at the Microsoft Community.
Apologies for any inconvenience you are experiencing. We are happy to help you.
Yes. You can't access that folder. This is by design. You won't want certainly that the virus should spread, hence the folder is protected.
Do these steps not help remove quarantined items?
Press the Windows key and type Windows Security.
Click on the Windows Security app from the search results.
In the Windows Security window, click on Virus & threat protection.
Under the Current Threats section, click on Protection History.
In the Protection history window, use the Filters drop-down menu to select Quarantined items.
Click on the quarantined item you want to remove.
A window will pop up; click Yes to confirm.
Click on the Action button and select Remove.
You can also use Remove-MpThreat CMDLET to remove the quarantined threats in Powershell.
Hope that helps, and rely on us for any further inquiries. All the best.