UPDATE. I checked inside the glovebox and effectively found/discovered the USB port.
Once I plugged in the USB stick in that USB port and clicked "Import journey (USB)", it started to download the itineraries which a forum fellow had kindly prepared (bar with increasing percentages). Subsequently the imported journeys appeared in the SATNAV menu. Checked these and these all work. Good job.
Conclusion: my mistake - I was using the wrong USB port for importing the SATNAV itineraries (inside the glove box = correct USB port | under the armrest = incorrect USB port).