Okay, replying to myself, as I just found this:
which I will try shortly, as looking at log files would be very helpful.
But in the meantime I printed an error message to screen and have another question: it IS the case that the deserialising of my json fails (FJsonSerializer::Deserialize), so I guess the file is missing. How do I get it copied over as part of the bundle? It’s just sitting in my Content directory at the moment.
Or in other words, all I did was drag a json file into $GAME_NAME/Content and now (unsurprisingly) this isn’t being copied to iOS. Can anyone tell me what I need to do to ensure this is bundled with the game?