Preheat the oven to 180 °C. Coat a bundt pan with the extra flour.
Finely grate the zest of the lemons and oranges. With a sharp knife, remove the peel and pips. Cut the lemons and oranges into segments and squeeze out the juice. Then put the segments in a separate bowl and cut into pieces.
2 Sift together the flour, sugar, baking powder and salt in a bowl. Add the sour cream and eggs, one at a time, beating to combine. Add the butter, and continue beating while adding the citrus juice and zest. Add the citrus pieces and beat just to combine.
3 Transfer the batter to the prepared pan. Bake for about 45 minutes or until a cake tester inserted in the middle comes out clean.
Bring the lemon juice and sugar to a boil in a saucepan, stirring until the sugar dissolves. 5 Pour the syrup over the cake and finish it off by dusting it with icing sugar.