After some more research into this, I think I've found the ultimate offline satnav option for Jolla: /2014/07/05/perfect-offline-map-solution-for-jolla/

Have a read and if you need a Satnav on your Jolla I would suggest giving this one (Mapfactor) a try. From initial testing it looks pretty good and offers either free (OpenStreetMap) or paid (TomTom) maps which you can select from within the app.

Chime up below on your experiences if you decide to give it a whirl. :-)