Heat the oven to 180C/160C fan/ gas 4. Grease and line a deep 20cm cake tin with baking parchment.
Mix the flour and cinnamon together in a large bowl. Add the butter and rub into the flour using your fingers, until it resembles fine breadcrumbs. Stir in the light brown sugar. Beat in the egg followed by 6 - 8 tbsp of milk – you want to achieve a smooth, thick batter.
Add the apples and sultanas and mix to combine. Scrape the batter into your prepared tin and gently level out. Sprinkle over the demerara sugar and bake in the oven for 30-40 minutes or until golden and a skewer inserted into the middle comes out clean.
Allow to cool in the tin for 15 minutes and then carefully turn out onto a wire rack to cool further. Best served still warm with a little custard.