OK I’ve worked it out now, for anyone else interested:
1. Get the dimensions of the original eg 1080×1920
2. In Project settings or Timeline settings window > Timeline resolution: – enter these values into the custom values fields
3. Do the edits etc
4. In the Deliver tab, select the preferred output eg YouTube 1080p
5. Enter custom resolution values as per imported/setup, eg 1080×1920
6. Render
These two setting ensure it imports and exports to the exact dimensions and orientation of the original.