Pressing play freezes but built game fine

Brand new to unity, using 5.3.1f1 on OS X elcapitan.

Got bare bones 'game' with 2 scenes: the main menu (entry scene) with UI button to open the next scene (which has just a UI text and UI 'back' button on it).

When i press Play in editor, clicking main menu button changes scene, but then the preview/emulator freezes. I cannot interact with the new loaded scene. I have not added any script/code of my own to run for new scene. If i press pause and play again, the UI buttons no longer work even on the main menu, i.e. the whole preview if frozen.

If i build the 'game' and run it on OSX, it behaves as expected. i.e. main menu button loads next scene, and I can interact with next scene clicking button to return to main menu etc.

Code for loading new scene from main menu:

 using UnityEngine;
 using System.Collections;
 using UnityEngine.SceneManagement;
 
 public class scriptMenu : MonoBehaviour {
 
     public void btnScene1_OnClick() {
         Debug.Log("<color=blue>Msg:</color> Loading scene1");
         SceneManager.LoadScene ("scene1");
         Debug.Log ("<color=blue>Msg:</color> Loaded scene1");
     }

What might cause the preview to freeze whilst the built game is fine ?

ta