Continuing to dabble with PowerApps.
Just discovered that you can change individual folder permissions in Sharepoint Groups Document Library using the Manage Access panel. I had been under the impression that to make changes like this you needed to drop into the Advanced Permissions settings ( something that most of the users in my organisation would not cope with easily ).
This Microsoft article shows how to do it. I will definitely be making use of this now I know about it, and helping my users to use it too.