If I put a simple dictionary such as this in my Xcode project:
var myDictionary:[String: String] = [
"a": "line 1",
"b": "line 2"]
I've solved this myself now (for the most part). It only happens when I debug using the iPhone 5 in the simulator. If I even move up to iPhone 5s the debugger works as I would expect.
This still leaves a bug where you can't debug properly on an iPhone 5 but this is not an issue for me and is something that i imagine Apple themselves would need to think about rather than me