Lay out a sheet of baking parchment and place the apples on this, close to your stovetop. Tip the sugar into a pan along with 100ml water and set over a medium heat. Cook for 5 mins until the sugar dissolves, then stir in the vinegar and syrup. Set a sugar thermometer in the pan and boil to 150C or 'hard crack' stage. If you don?t have a thermometer you can test the toffee by pouring a little into a bowl of cold water. It should harden instantly and, when removed, be brittle and easy to break. If you can still squish the toffee, continue to boil it.