Line a 15cm tin with baking parchment. Put the sugar, golden syrup, butter and water into a large heavy-based pan and set over a medium heat. Stir until the sugar has dissolved then turn up the heat and bring to the boil. Cook, without stirring, until the mixture reaches 138°C (you can use a candy thermometer to check this) then remove the pan from the heat.
If you don't have a confectionery or sugar thermometer, you can test the mixture by dropping a small amount into cold water; if it is ready, it will become a hard toffee ball. This is called hard crack stage.
Taking care, add the vinegar and bicarbonate of soda to the pan – the mixture will bubble up and rise. Immediately pour into the lined baking tray; leave for 15-20 minutes to set then, using a sharp knife, score into squares. When completely set, break along the scored lines, remove from the greaseproof paper and store in an airtight tin for up to two weeks.
You can make your bonfire toffee even more special by dipping it into melted chocolate. The crunchy honeycomb with a dark chocolate coating is delicious!