Place the caster sugar, butter and zest of the lemon and orange in a bowl and beat together until pale and creamy.
Add the milk and then beat in the eggs, one at a time, with a spoon of flour to prevent the mixture from curdling. Mix in the remaining flour. Spoon into the prepared tin/tins and level the surface flat.
Preheat oven to 180 C.
Grease and line the base of 1 large (900 g) or 2 small (450 g) loaf tins.
Bake in the oven for about 45 minutes for a large cake or 30 minutes for a smaller cake until golden brown or cooked (the skewer should come out clean when inserted into the centre of the cake).
While the cake is cooking, place the lemon and orange juice in a saucepan, bring to the boil and allow to reduce in quantity to about 3 tablespoons. Leave to cool, and then stir in the granulated sugar so it just starts to dissolve.
As soon as the cake comes out of the oven, prick several times with a skewer, and slowly pour the lemon-orange sugar syrup all over the top, letting it soak into the cake. Leave to cool completely in the tin before turning out and cutting into slices.