NullReferenceException inside clone script

I keep getting the same error on this and it's had me stumped for hours. Essentially I instantiate an object called "unit", which has a script called movement which instantiates "head" (in the start function). Except that it doesn't instantiate it, the following code gives this.

 public class Movement : MonoBehaviour { //Movement is the behaviour which allows a unit to move
     
     public Transform HeadPrefab;
     public Transform Block;
     public int MaxSize = 5;
     
     Transform head;
     List<Transform> blockList = new List<Transform>();
     
     void Start () {
         
         Vector3 headPosition = new Vector3(0,5,-11);    
         
         head = Instantiate (HeadPrefab, headPosition, Quaternion.identity) as Transform;
         Debug.Log(head.transform.position.x);
     }
 }

gives: NullReferenceException: Object reference not set to an instance of an object. at runtime, when compiled there are no errors.

The unit object is instantiated by a script called GameArena and is as follows

     public Transform unit;
     public Transform enemy;
     
     Transform clone;
     
     void Start () {
         
         Vector3 unitPosition = new Vector3(0,5,-11);
         Vector3 enemyPosition = new Vector3(10,5,-11);        
         
         clone = Instantiate (unit, unitPosition, Quaternion.identity) as Transform;
         Instantiate (enemy, enemyPosition, Quaternion.identity);
         Debug.Log(clone.transform.position.x);
         Debug.Log(clone);
     }

again no errors at all on this and it reports to the console the correct position of 0.

I'm confused that I seem to be doing the same things twice but it only works one time, what's different in "Movement"?