Peel, core, and chop apples into 1–2cm chunks. Toss immediately in lemon juice. Set aside.
Preheat oven to 180°C / 160°C fan / Gas 4. Grease and line a deep 20cm round tin.
Beat butter and caster sugar together for 4–5 minutes until pale and fluffy. Add vanilla extract.
Add eggs one at a time, beating well after each. Add a spoonful of flour if the mixture curdles.
Sift in flour, baking powder, cinnamon, and mixed spice. Add milk. Fold gently until just combined.
Fold in apple chunks until evenly distributed.
Spoon into prepared tin and level the top. Mix demerara sugar and cinnamon together and scatter generously over the surface.
Bake for 50–60 minutes until deep golden and a skewer comes out clean. Cover top with parchment if browning too quickly.
Cool in tin for 15 minutes. Turn out and serve warm or at room temperature.