Preheat the oven to 180°C/gas mark 4. Grease and line a 3lb loaf tin.
Beat together the butter and sugar until they are light and creamy. Add the eggs one by one, mixing well. You may want to add a spoonful of flour after each addition to prevent the eggs from curdling. Sift in the flour and baking powder, then gently fold until the flour is incorporated. Add the milk and lemon zest and mix well. Spoon the mixture into the prepared tin and bake in the oven for 1 hour, until the crust is golden and a skewer comes out clean when inserted.
Meanwhile, mix together the granulated sugar and lemon juice. Remove the cake from the oven, prick it all over with a cake skewer, then pour over the lemon and sugar mixture. Leave to cool completely in the tin, then remove and slice to serve.