So says Andrew Morton. But I don't know how different "Countess Spencer held a very special place in Diana's heart and she sincerely believed that her grandmother looked after her in the spirit world" is from orthodox Christian beliefs about loved ones looking down on someone from heaven. We don't know where Diana's belief ends and Morton's interpretation begins. Also he doesn't "time stamp" his claim so it's hard to tell if this was Diana's adult belief or something she held onto as a child.
I have no idea what Andrew Morton said; I didn’t get it from him.