Preheat the oven to 180°C/350°F/gas 4. Put the soft butter into a mixing bowl and beat with a wooden spoon or electric mixer for 1 minute or until creamy. Gradually beat in the sugar, then continue beating until the mixture becomes paler and fluffy.
Gradually add the eggs, beating well after each addition; add a tablespoon of the flour with the last portion of egg. Sift the remaining flour, the salt and baking powder into the bowl and gently fold into the mixture with a large metal spoon. When thoroughly combined add the marmalade and milk and stir in.
Spoon the mixture into the prepared tin and spread evenly. Bake for 50 to 55 minutes or until a good golden brown and firm to the touch. Run a round-bladed knife around the inside of the tin to loosen the cake, then carefully turn out onto a wire rack. Gently warm the second portion of marmalade and brush over the top of the warm cake. Leave to cool completely.
Sift the icing sugar into a bowl, add the warm water and mix to a smooth, runny icing using a wooden spoon. Spoon the icing over the cake and let it run down the sides – the chunks of marmalade will stick up through the icing. Leave until set before cutting. Store in an airtight container and eat within 5 days.