It's very odd - with some older images, my application falls over the first time I attempt to load it, but works subsequently. I've added error handlers everywhere, but still can't catch it. My recent images work fine.
However, and this is a big however, I'm writing a similar application using Swift, so it's a native app. A very similar issue exists, only the app doesn't crash, it just returns a nil image.
This leads me to believe it's an issue with iOS 8 and *not* with AIR.
So, John and Pravendra, it looks like I was barking up the wrong tree! My apologies and thanks to you both for your help,
Simon