Sieve flour into a large bowl, add butter (or marg) and caster sugar. Whisk until creamy (I used an electric whisk for quickness). Put the mix into the cake tin a spread evenly. Cut up the apple into thin slices and add on top of the cake. Mix the cinnamon powder and demerara sugar together and sprinkle over the top of the cake.
Bake in the preheated oven for 10 to 15 minutes or until risen and firm and a skewer inserted into the centre comes out clean. Remove from the oven and allow to cool in the tin for 10 minutes, before transferring to a cooling rack to cool completely.