Place diced apples in a medium sized saucepan, add a splash of water, and cook them with the lid on until soft (10-15 minutes). A few lumps of apple is fine to add a bit of texture
Stir the sultanas into the apples and leave to cool.
Preheat the oven to 180°C / 160°C fan assisted / gas mark 4 / 350°F
and grease and line a 20cm square cake tin (including up the sides of the tin).
Weigh out 200g of the oats and tip them into a food processor. Pulse them gently so they break down into a finer oat crumble, but not as powdery as oat flour. Toss into a large mixing bowl with the rest of the oats, the ground cinnamon and the salt. Set aside for a moment.
Melt the butter in a medium pan over a low heat then add the golden syrup and sugar.
Stir together until combined then remove from the heat and pour into the oat mixture, stirring well until combined.
Spoon half of the buttery oat mixture (425g) into the tin and press down evenly.
Pour over the Bramley apple mixture into an even layer, but don’t go right to the edges.
Spoon the rest of the flapjack mixture evenly over the top to cover all the apples. Crumble the nuts over the top and press the topping together so that it seals in the apple as much as you can.
Bake in the oven for 40 minutes.
Remove from the oven and leave to cool completely in the tin placed on a wire rack.
Once cool, remove from the tin and cut into squares.