Preheat oven to 180ºC / 160ºC Fan / 350ºF / Gas Mark 4. Line a 2lb loaf tin with parchment paper and set aside.
To make the cake, place the softened butter in a large mixing bowl. In another small bowl, add the sugar along with the orange zest and lemon zest. Using your fingertips, rub the sugar and zest together until combined.
Add the sugar/zest mixture to the bowl containing the butter. Cream for a couple of minutes, until light and fluffy. Now add the eggs one at a time, beating well after each addition. Don't worry if the mixture looks slightly curdled at this point, as once the flour is added it'll be fine.
Now sift in the flour and gently fold in until completely incorporated. Transfer the cake batter to the loaf tin and spread out until evenly distributed.
Bake the cake for 55-60 minutes, or until a cake tester comes out clean when inserted into the centre of the cake.
Now prepare the sugar syrup mixture. In a small bowl, combine the sugar and the orange and lemon juice. Stir together until the sugar has dissolved. Pour this syrup all over the cake - it will soak into the cake as it cools.
Once the cake has cooled, remove it from the tin and place on your chosen serving dish/plate. Make the icing by whisking the icing sugar and enough orange/lemon juice until you have the drizzle consistency you desire.
Drizzle the icing over the cake and down the sides and decorate further with extra orange and lemon zest. The cake will keep, covered at room temperature for up to 3 days.