Heat the oven to 180C/fan160C/gas 4. Beat the butter, sugar and oil in a stand mixer until the mixture looks light and fluffy. Beat in the lemon zest (keep the juice), followed by the eggs. Sift the flour and baking powder together. Fold this into the cake mix, adding the buttermilk as you do so. Fold until the mixture is smooth. Fold through the redcurrants. Spoon into a buttered and floured bundt tin or cake tin. Bake for one hour, then check that the sides of the cake are starting to shrink away from the edges and the top is pale brown. Cool in the tin for a few minutes, then turn out onto a wire rack.