Line a baking sheet with parchment paper.
Skewer each apple firmly with a lollipop stick and place on the sheet.
Put the sugar, butter, syrup, vinegar and 120ml water in a large, heavy-based non-stick pan.
Heat, stirring, for 5 minutes until the sugar has dissolved.
Bring to the boil and boil rapidly for 5-7 minutes.
Drop a tiny bit of the mixture into a bowl of ice-cold water – if it forms a firm ball, the toffee is ready.
Remove the pan from the heat and, working quickly, take each apple by its stick and dip it into the toffee, swirling to coat evenly.
Allow the excess toffee to drip off, then put back on the baking sheet to cool and set.