Currently my small game is running perfectly in the Unity Engines testplay section, though things act differently when I build the game and run it.
The issue that occurs is, in the build, when my character runs out of lives it should change scene from the game scene to the menu, however, it just continues in the game scene and some things continue to run while others break and do not.
Any reasons why a build would react differently to the Application.LoadLevel() call?
Did you add your lvl’s in the build settings. I’m sure you did but thats the most common error. 
That was my exact thought. Sounds like you didn’t export your levels.
All of them are added, I’ve checked many times and all boxes are checked. Any other common (or otherwise) reasons?
Run in debug mode and see what it’s trying to do when the player dies and it tries to load the menu. It’s probably loading something that it can’t find. Named wrong, or the wrong id.