Tip the apples, demerara sugar and brandy into a pan, bring to the boil and simmer for 12-15 mins until caramelised and sticky, then leave to cool. Scoop about a third of the wedges out of the sauce and set aside, then use a blender or food processor to purée the rest of the apples and sauce, then set aside.
Separate the eggs and tip the yolks into a large metal bowl. Set aside the whites for later. Dissolve 100g of the caster sugar in a medium saucepan with 120ml boiling water. When clear, bring to the boil and put a sugar thermometer in the pan. Meanwhile, whisk the yolks in a bowl with an electric mixer until creamy. When the sugar syrup reaches 120C, remove from the heat at once. With the mixer running, drizzle the syrup onto the yolks and beat, on full speed for 3-5 mins until you have a firm yellow foam. Leave to cool, whisking occasionally.
Whisk the egg whites in a clean bowl with a clean whisk until they form stiff peaks, then whisk in half of the remaining caster sugar and continue to whisk for 30 seconds, then add the rest of the sugar and continue whisking until stiff peaks have formed again. In a third bowl, whisk the cream until soft peaks form. Gently fold the apple purée through the egg yolk mixture, then fold that through the egg whites and lastly fold that mixture through the whipped cream. Keep on folding until everything is fully combined. Pour into a 900g loaf tin lined with cling film and freeze overnight. Can be made two weeks in advance and kept frozen.
To make the sauce, tip the sugar and butter into a saucepan and heat until it starts to melt. Once the sugar begins to bubble, add the cream, water, apple juice and cinnamon, cook out for a minute, then cool to room temperature. When ready to serve, turn out the parfait, top with the reserved apple wedges and the digestive crumbs, then drizzle over the toffee sauce.