Preheat the oven to 180 degrees C. Generously butter and flour the 23cm springform cake tin, tapping out any excess flour. Set aside.
In the bowl of an electric mixer fitted with a whisk, beat the eggs and sugar until thick and lemon-coloured, about 3 minutes. Add the butter, oil, milk and vanilla essence and mix until blended.
Sift the flour, baking powder and salt into a large bowl. Add the lemon zest and orange zest and toss to coat the zest with flour.
Spoon the flour mixture into the bowl of batter and stir with a wooden spoon until thoroughly blended. Scrape down the sides of the bowl and mix once more. Set aside for 10 minutes to allow the flour to absorb the liquids.
Stir in about 3/4 of the grapes into the batter. Spoon the batter into the prepared cake pan and smooth out the top with a spatula.
Place the pan in the centre of the oven and bake for 15 minutes, then sprinkle the top of the cake with the remaining grapes. Bake for about 40 minutes more, until the top is a deep golden brown and the cake feels quite firm when pressed with a fingertip.
Remove from oven and cool on a wire rack. After 10 minutes run a knife along the sides of the pan. Release and remove the side of the springform pan, leaving the cake on the pan base. Serve at room temperature, cut into thin wedges.