Place the apples in a large bowl, then cover with boiling water (you may have to do this in 2 batches). This will remove the waxy coating and help the caramel to stick. Dry thoroughly and twist off any stalks. Push a wooden skewer or lolly stick into the stalk end of each apple.
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 140C 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.
Working quickly and carefully, dip and twist each apple in the hot toffee until covered, let any excess drip away, then place on the baking parchment to harden. You may have to heat the toffee a little if the temperature drops and it starts to feel thick and viscous. Leave the toffee to cool before eating. Can be made up to 2 days in advance, stored in a dry place.