Hi @Marc Lodge ,
We can't use UNC to access files that exist on SharePoint through a browser because modern browsers don't support the file:// protocol.
You could try creating Network Mapping Drive as a workaround via a UNC-like WebDav path, but it is still not recommended. Because this is legacy technology, it may be affected by the August 2021 Internet Explorer deprecation. The mapped network drive needs to be maintained after it is established, every two weeks by navigating to the SharePoint library in Internet Explorer, refreshing the "security token" via Open with explorer to get online permissions.
This is a specific reference about creating: Map a network drive to a SharePoint library
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