In a small bowl, stir oats into boiling water. Set aside to soak for 20 minutes.
Preheat the oven to 180 degrees C. Grease a 20cm square baking pan.
In a medium bowl, cream together the white sugar, 1 cup brown sugar and margarine until smooth. Beat in the eggs one at a time. Sift in the flour, bicarb soda, salt and nutmeg; stir just until moistened. Mix in the soaked oats. Pour into the prepared pan and spread evenly.
Bake for 35 to 40 minutes in the preheated oven, until a toothpick inserted into the cake comes out clean. Turn the oven to the grillsetting and let it heat up.
In a small bowl, stir together the melted butter, 1/2 cup brown sugar, heavy cream, coconut and vanilla. Spread over the top of the cake. Grill for a few minutes, just until the coconut is lightly browned.