OK so after a decent amount of messing around with build settings, the original problem is solved (i.e. no more Lexical or preprocessor errors)
After fixing the build settings, and refactoring any code that had become outdated (which was a lot) I now have 9 Linker Errors.
Can anyone tell me where to go from here?
UPDATE: The Linker errors only appear when building for the simulator, building for a device is fine.