Peel and grate the apple.
Put the sugar and apple juice in a large, heavy-based saucepan, then heat until the sugar dissolves.
Add the grated apple and all other ingredients except the chestnuts, brandy and lemon.
Cook for 20-25 minutes until the fruits are soft and the sauce is thick and sticky.
Add the chestnuts and 50 ml brandy and cook for 5 more minutes, stirring, until you get a thick, jam-like mixture. Turn off the heat and add the remaining ingredients.
Stir well, then add the extra brandy (for a really boozy version) and spoon it into 2 sterilised jars.
When cooled completely, put the jars in the fridge. or in a cool larder. Once opened, store in the fridge.