Preheat the oven to 180°C/160°C fan/350°F/gas mark 4. Grease an 18 cm / 7 in round, deep cake tin and line the base with baking parchment.
Core and dice the apples (peel them if you want), set aside.
Mix together the flour, salt, cinnamon and mixed spice in a large bowl. Add the butter and rub in using your fingertips until the mixture resembles course sand and no lumps of butter remain.
Stir in the sugar, then the apples. Mix in the milk to form a batter. The mixture should be quite stiff and may look a bit curdled, that is fine.
Scrape the mixture into the prepared tin and spread it level. Bake for about 1 hour, but start checking for doneness after 45 minutes. It should be firm and a skewer inserted into the centre should come out clean. Cover loosely with foil partway through baking if the top begins to darken too much.
Remove the cake from the oven and immediately sprinkle with demarera sugar. Leave to cool in the tin for 20-30 minutes then turn out onto a wire rack and leave to cool completely. Store in an airtight container.