Preheat oven to 180 degrees C. Grease and flour a 23cm round pan. Sift together the flour, baking powder, bicarb soda and salt. Set aside.
In a large bowl, cream together the margarine and 1 cup sugar until light and fluffy. Beat in the eggs one at a time. Beat in the flour mixture alternately with the sour cream, mixing just until incorporated. Pour half of the batter into prepared pan. Mix together 1/4 cup sugar and 1 teaspoon cinnamon. Sprinkle half of mixture over batter in the pan. Cover with remaining batter, then top with remaining cinnamon sugar.
Bake in the preheated oven for 35 to 45 minutes, or until a toothpick inserted into the centre of the cake comes out clean. Allow to cool.