it's in the app itself, so my mistake. but whilst in the app itself click on settings>>more>>cache settings there is a 'pre-fetch' even when not on wi-fi option. and also a clear cache. my cache is currently just over 1mb so i'm guessing that is the last journey i did.
i think if you click that and configure your journey, even if you are not on wi-fi it will start to pre-fetch and cache your journey so that if you loose network coverage during your journey, you will be safe.
obviously this is a different concept to traditional sat nav where the maps are pre-loaded, so for instance if you lost network coverage and wanted to program a new route once you'd reached your initial destination then you'd be stuck!