Place fruit, butter, sugar, brandy and the water in a medium saucepan; stir over medium heat until butter is melted and sugar is dissolved. Bring to the boil. Remove from heat; transfer to a large bowl. Cool to room temperature.
Preheat oven to 150C/300F. Line base and side of a deep 20cm (8 inch) round cake pan with three layers of baking paper 5cm (2 inch) above the side.
Stir rinds, treacle and egg into the cooled fruit mixture, then stir in sifted dry ingredients. Mix until just combined. Spoon cake mixture into the prepared pan.
Bake cake for about 2 hours. Cover hot cake tightly with foil and cool in the pan overnight.
To prepare icing, combine icing sugar and water and mix until combined. Pour over cake, allowing some to drip down the sides. Decorate with berries and rosemary.