Heat oven to 180C/160C fan/gas 4. Grease a 900g/2lb loaf tin and line the base and ends with a long strip of baking parchment. Put the butter, sugar, flour, eggs and vanilla extract into a large bowl and beat with an electric hand mixer for 5 mins until pale and creamy - the mixture will be very thick.
Spread one-third of the cake mix into the tin, then scatter over 50g of the fruit. Carefully dot and spread another third of the cake mix on top, and scatter with another 50g fruit. Finally dot the rest of the cake mix over and gently spread with the back of a spoon. Bake for 1 hr, until an inserted skewer comes out clean.
Poke the cake all over with a skewer. Put remaining 75g fruit into a bowl with the granulated sugar. Stir in 1 tbsp of the citrus juice first with a fork, mashing a little of the fruit as you go. If it’s a bit dry, add a splash more juice and spoon over the cake. Leave in the tin until the cake is cool and the topping is set and crisp.