Preheat oven to 160°C (fan)/ 320°F/gas mark 4. Butter a 15cm/6" loaf tin, and line with baking parchment/silicone paper.
Beat the butter with a wooden spoon or electric mixer until creamy. Add the sugar and beat until the mixture lightens in colour and becomes fluffy
Beat in the eggs. Mix together the flour, salt and baking powder well in a separate bowl, and add to the butter, sugar and egg mixture. Fold in the flour. Add the marmalade and milk, and fold in as well.
Spoon the mixture into the loaf tin and spread. Bake for about 50 – 55 mins until golden brown and firm to the touch, and when a cocktail stick is poked into the cake it comes out clean. Allow to cool for 5 minutes then run a round blade knife round the inside of he tin and turn out onto a wire rack.
Sift the icing sugar into a bowl, add the warm water and mix. Stir in the marmalade. Spoon the mix over the cake, letting it run down the sides. Leave to set before cutting.