Preheat oven to 170 C / Gas 3. Grease and flour a 23cm tube cake tin. Sieve together the flour, baking powder, bicarbonate of soda and salt. Set aside.
In a large bowl, cream together the butter and 400g sugar until light and fluffy. Add the eggs one at a time, beating well with each addition, then stir in the vanilla. Add the flour mixture alternately with the soured cream; beat well. Spread half of the batter into the prepared tin. Mix together the cinnamon and 200g sugar. Sprinkle most of it over the batter in the tin. Spread the rest of the batter into the pan and sprinkle any remaining sugar mixture over the top.
Bake in the preheated oven for 60 to 70 minutes or until a skewer inserted into the cake comes out clean. Allow to cool for 10 minutes before inverting onto a wire rack to cool completely.