Heat the oven to 180C/350F/Gas Mark 4. Beat the butter or spread with the sugar in an electric mixer for 3-5 minutes or until it is light and fluffy. Grate the rind from the lemon. One at a time lightly beat in the eggs with a tablespoon of flour then fold in the rest of the flour along with the lemon rind, the poppy seeds and the juice of ½ a lemon. Line an 18cm/7inch loaf tin with oiled nonstick baking paper. Spoon the mixture into the tin and tap the cake tin gently a few times on the counter top to flatten out the top. Bake for 45 minutes or until a skewer comes out clean. Remove from the oven, allow to cool for a few minutes and then remove from the tin onto a rack and allow to cool completely. When the cake is cold, put the icing sugar in a small bowl and stir in up to 5 tsp of lemon juice - you want the icing to be quite thick but spreadable. Spoon it onto the top of the cake and then smooth it out with a spatula dipped in hot water, allowing the drips to run down the side of the cake. Leave icing to set before cutting.