Restrict shareing files in a document library to members of that document library

Hi ian-mcguinness​ 

it is not possible to restrict sharing links to files to only users that already have access to the document library by default. 

Here are a lot of documentation, but i think not what you looking for: 

https://learn.microsoft.com/sharepoint/turn-external-sharing-on-or-off?WT.mc_id=DX-MVP-5004845 

You can also set the Site sharing settings, but not restricted to existing users:

As an alternative, you can try to use a flow, checks whether the user is authorized or not.

Best, Dave