Preheat the oven to 190°C/Fan 170°C/gas mark 5.
Start by making the filling. Put the apples, golden caster sugar, cinnamon, lemon juice and plain flour in a saucepan with 50ml water. Stir well and cook on a medium heat for roughly 15 minutes until the apples have softened nicely but are still in distinct apple pieces, stirring all the time.
For the topping, combine the crumble ingredients - oats, demerara sugar, self-raising flour and chopped hazelnuts - in a large bowl. Pour over the melted butter and vanilla extract and stir to combine.
Once the apples have softened and the topping has been combined, lightly grease an ovenproof dish (roughly 28 x 25cm and at least 5cm deep) with a little butter.
Fill the dish with the softened apples and top with the crumble mixture, making sure all the apples are completely covered.
Slide onto the middle shelf of the oven and bake for 40 minutes or until you have a lovely golden topping.
To serve, top the apple crumble with vanilla ice cream or custard, or serve with double cream.