Preheat the oven to 170 degrees Celsius (340 Fahrenheit).
In a pan set over a medium heat, add the butter cut into cubes, dark brown sugar, black treacle and golden syrup, and use a spatula to mix well until the butter is melted, and you have a smooth mixture.
In a large mixing bowl, sift the flour, add the bicarbonate of soda and spices, and mix really well.
Beat the egg and add it to the flour together with the milk and cooled butter mixure.
Use a spatula to mix well until you have a smooth batter.
Butter and flour a loaf tin, and spread the batter evenly.
Bake for 50 minutes or until a skewer inserted in the middle comes out clean.
Remove the cake from the oven, and allow it to cool down slightly in the loaf tin.
Run a sharp knife around the edges of the loaf, the cake will come out easily.
Leave it to cool down completely on a cooling rack.
To make the icing, sift the icing sugar, add half a tablespoon of the lemon juice, and mix well to get a slightly runny smooth mixture, adding more lemon juice if needed.
Spread the icing over the cake, and top with lemon zest.