Ok I got it working. Not entirely sure how, but this is what I did:
I had previously set up an ‘aggregate device’ setting in OS/Setting?Sound. This AD included the external mic.
So in Premiere/Prefs/Audio Hardware, I toggled the input audio from the Ext Mic, to the Aggregate Device; then toggled to anything else, then toggled back to Ext Mic.
This did it. Again, not sure why. But it all works now. Hope that info helps someone.