Pour the wine into a small pan. Bring to the boil, then keep simmering until reduced down to 85ml - this will take 5-10 mins. Leave to cool.
Heat oven to 180C/fan 160C/gas 4. Brush a 23cm springform cake tin with olive oil, tip in 1 tbsp flour, then shake all over the pan until covered. Discard any excess. Beat together the sugar and butter until creamy and well combined. Add the eggs, one at a time, then stir through the zests. Stir together the cooled wine and olive oil, then pour a little into the cake mix. Stir, then fold in a third of the flour and the baking powder. Keep alternating between adding the liquid and flour until everything has been incorporated and the batter is smooth.
Spoon the cake batter into the prepared tin, then smooth the surface with the back of the spoon. Scatter the halved grapes over the top, then sprinkle over the sugar. Bake for 35-40 mins until a skewer inserted into the middle of the cake comes out clean. Eat warm or cool. Will store in an airtight container for up to 3 days.