Begin by preheating the oven to 180C/ 250F/ GM4 and grease & line a 20cmx30cm baking tin.
Next mix the spices together in a bowl.
Put the demerara sugar into a small bowl, add 1 teaspoon of the mixed spice and stir.
Sieve the flour, baking powder, baking soda, salt and the remaining spices into a bowl.
Prepare the apples by cutting each one into eight wedges. Remove the core, then slice each wedge into chunks approximately ¼cm thick. Put into a bowl and toss with the lemon juice to prevent browning.
Using electric beaters, cream the butter and sugar together until pale and fluffy.
Add the eggs, one at a time, beating well between each addition. If the batter splits, mix in a spoonful of flour to help bind the ingredients back together.
Mix in the soured cream.
Sieve the flour mix into the batter and beat, on medium speed, until combined and the batter is smooth. Take care not to overbeat the mixture, as this will lead to a tough sponge.
Spoon the batter into the baking tin and use a blunt knife to spread it out evenly.
Drain the apple from the lemon juice and scatter over the top of the batter.
Reserve 1½ teaspoon of the demerara sugar and sprinkle the rest evenly over the cake.
Transfer to the oven and bake for around 30 minutes until the cake is golden, well risen and a skewer poked into the centre comes out clean.
Once baked, remove from the oven and sprinkle the remaining sugar over the top. Leave to cool completely in the tin.
Carefully take out of the tin and remove the baking paper. Cut into 15 pieces using a serrated knife and sharp, clean scissors to cut through any awkward pieces of apple.
Wrap in foil and store in an airtight container for up to 5 days