Preheat oven to 180 C / Gas 4. Grease and flour a 23cm tube cake tin.
In a large bowl, cream together 200g sugar and butter. Beat in the eggs, one at a time. Combine the flour, baking powder, bicarbonate of soda and salt, stir into the creamed mixture. Finally, stir in the water, vanilla and soured cream.
In a small bowl, combine the walnuts, remaining sugar and cinnamon. Pour half of the cake batter into the prepared tin. Sprinkle half of the nut mixture over it, then top with the remaining batter and sprinkle with remaining nut mixture.
Bake for 50 to 60 minutes in the preheated oven. Cool in tin for 10 minutes, then invert onto a wire rack to cool completely.