He had arrived ten minutes too late to say goodbye. He had received the telephone call, set the co-ordinates but arrived ten minutes too late. Ten minutes. He could've gone back in his timelines to get there earlier but it wouldn't have been right. She was already gone, he wasn't going to make her go again purely for his benefit.
He knelt by her bedside, where she lay peacefully, an expression of serenity upon her youthful, but tired face. It looked like she was sleeping, and he tried to imagine that she was. He stroked a piece of her hair and tucked it behind her ear. He put a new bunch of lilies, her favourites, in the vase next to her bedside. He kissed her on the forehead, sat in the chair beside her beautiful, but broken, body and cried. He wept for his beloved Sarah, his most faithful and loving companion. The woman who had turned him from angry and cold to cheery and caring; he just couldn't believe that she was gone. He sobbed for hours until a doctor came in and told him that would he please leave as visiting hours were over, and he offered his deepest sympathies.
He skipped the next few days and went straight to the funeral. There, Sarah Jane's body was burned into a billion tiny ashes and placed in a pot, with messages written all over it. The next day is bonfire night and Sarah's wish will come true at last.
The firework takes precious few seconds to fly hundreds upon thousands of metres in the black night sky. It explodes ferociously, huge shafts of bright beaming light spreading across the Earth. And up there, in the beauty of this primitive, but extraordinary, firework, Sarah Jane Smith finally goes to sleep, her ashes spreading over the world and out into the universe that she loved. It's what she wanted, to be remembered and to be scattered, so she could watch over her loved ones and watch over him, her marvellous Doctor.