UNET Custom Spawn Functions never called. Tearing my hair out

Hi, I am attempting to implement some custom spawn handlers as described in :

https://docs.unity3d.com/2018.1/Documentation/Manual/UNetCustomSpawning.html

However I cannot for the life of me get any of my spawning functions to be called. In all related threads this issue is caused by the prefab to be spawned being registered with the network manager. However i have confirmed this is not the case and have tried with many different prefabs.

Here is my spawner code which is a stripped down version of the example given in the docs :

 using UnityEngine;
 using UnityEngine.Networking;
 using System.Collections;
 
 public class SpawnManagerExample : MonoBehaviour
 {
     public GameObject m_Prefab;
     
     public NetworkHash128 assetId { get; set; }
 
     public delegate GameObject SpawnDelegate(Vector3 position, NetworkHash128 assetId);
     public delegate void UnSpawnDelegate(GameObject spawned);
 
     public GameObject newPlayer;
 
     void Start()
     {
         assetId = m_Prefab.GetComponent<NetworkIdentity>().assetId;
         newPlayer = (GameObject)Instantiate(m_Prefab, Vector3.zero, Quaternion.identity);
         
 
         ClientScene.RegisterSpawnHandler(assetId, SpawnObject, UnSpawnObject);
     }
 
     public GameObject GetPlayer()
     {
         Debug.Log("GETTING PLAYER");
         return newPlayer;        
     }
 
     public GameObject SpawnObject(Vector3 position, NetworkHash128 assetId)
     {
         Debug.LogError("CUSTOM SPAWNER CALLED");
         GameObject newGameObject = GetPlayer();
         newGameObject.transform.position = new Vector3(5.0f, 5f, 5.0f); 
 
         return newGameObject;
     }
 
     public void UnSpawnObject(GameObject spawned)
     {
         Debug.Log("Re-pooling GameObject " + spawned.name);
         spawned.SetActive(false);
     }
 }

The RegisterSpawnHandler method is definitely being called as confirmed by networkmanager debug logs:

RegisterSpawnHandler asset 'e0727344931b63b428c654250030de52' SpawnObject/UnSpawnObject UnityEngine.Networking.ClientScene:RegisterSpawnHandler(NetworkHash128, SpawnDelegate, UnSpawnDelegate) SpawnManagerExample:Start() (at Assets/SpawnManagerExample.cs:22)

And this is the part of a script that calls the spawn:

    private SpawnManagerExample spawnManager;
 
     void Start()
     {
 
         spawnManager = GameObject.Find("SpawnManager2").GetComponent<SpawnManagerExample>();
 
 
     }
 
     private void Update()
     {
         if (splitscreenAmount < 1)
         {
             if (Input.GetButtonDown("CreateAnotherLocalPlayer") && isLocalPlayer)
             {
                 
                 CmdSpawnSplitScreenPlayerWithClientAuthoity();
 
                 splitscreenAmount++;
             }
 
         }
         
     }
 
 
     [Command]
     private void CmdSpawnSplitScreenPlayerWithClientAuthoity()
     {
         var newPlayer = spawnManager.GetPlayer();
 
         NetworkServer.Spawn(newPlayer, newPlayer.GetComponent<NetworkIdentity>().assetId);
     }

I have attempted many different variations of the NetworkServer.Spawn method and have ensured assetIds match with the registered spawn handler, but the SpawnManagers spawnObject method is never called.

Strangely the SpawnManagers unSpawnObject is called when the server is shutdown, but not by using the NetworkManager.unSpawn method .

I've now tried everything i can think of so any help would be much appreciated :(