I eventually solved the problem. I had a relationship, just the one, that I hadn't marked as optional. My tip to anyone else who has a similar problem is to make sure you run the app on the simulator rather than just directly on your phone as the debug information will give you a much clearer idea of what is going wrong. I'm just a hobbyist writing my first app so whereas proper developers would have known this, I didn't have a clue! My app is looking fabulous now though :)