Nothing in the console, but I got a message against the script itself saying that the script either did not contain a MonoBehaviour or the class name did not match the filename.
I think part of the issue was that i was creating a variant of the Block script in Block breaker because i was experimenting with some functionality, and at the same time was creating some new prefabs that referred to some scripts that had been partially edited.
I have archived the closed script and remapped the prefabs and the problem has gone away. I have some different problems now, but I think they are covered in the course: mainly game objects not allocated in the Inspector.