It seems the key line in the doucmentation that I missed is as below:
To remove an app from the private store
Sign in to the Microsoft Store for Business or Microsoft Store for Education.
Click Manage, and then choose Products & services.
Find an app, click the ellipses, choose Remove from private store, and then click Remove.
Choose the private store collection, and then under In collection, switch to Off.
The app will still be in your inventory, but your employees will not have access to the app from your private store.
I think therefore I cannot fully remove it from our inventory, only remove it from view in the private store?
Many thanks.