Heat the oven to 180C/fan 160C/gas 4. Butter and line the base of a loaf tin roughly 17x8x10cm. Beat the butter and caster sugar using an electric whisk until pale and creamy. Add the lemon zest and whisk the eggs in little by little, beating well between each addition - don’t worry if it looks curdled.
Fold the flour and almonds into the cake mixture using a large metal spoon or spatula, and then fold in 2 tbsp lemon juice. Spoon into the cake tin and bake for 50 minutes or until a skewer inserted into the middle of the cake comes out without any raw mix on it. Cool for 10 minutes.
Meanwhile, mix the remaining lemon juice and enough icing sugar to make a thin icing. Poke a few holes into the top of the cake with a toothpick and drizzle some of the icing over the cake. Cool in the tin for a further 30 minutes and then remove from the tin, put on a rack set over a tray and drizzle on the rest of the icing. Cool completely.