Preheat the oven to 170°C (325°F) or 150°C (300°F) for fan ovens.
Butter a 9 x 5 x 3 inch (23 x 13 x 7 cm) loaf pan. You can line it with baking parchment too if you like.
Mix the self-rising flour and the all-purpose flour together in a medium bowl and set aside.
Put the softened butter and sugar in a large bowl or the bowl of an electric mixer and cream them together either with a wooden spoon or the mixer. (Cream means to beat together thoroughly, scraping the bowl down regularly. It will take about 4 minutes in an electric mixer.)
Stir the lemon zest into the creamed butter and sugar.
Add the eggs to the butter and sugar mixture one at a time, adding one tablespoon of the flour mixture with each egg to prevent the mixture from curdling. Beat thoroughly between the addition of each egg.
Gently fold in the remaining flour.
Stir in the lemon juice and vanilla or almond flavouring, if using.
Transfer the batter into the loaf pan and sprinkle the top of the batter with a tablespoon or so of sugar.
Bake for 55 to 60 minutes or until a skewer inserted in the centre of the cake comes out clean.
Place the loaf pan on a wire rack to cool completely before removing the cake from the pan.