Put the syrup, sugar, butter and water into a heavy-bottomed pan. Stir over a medium heat until the sugar has dissolved.
Turn up the heat and bring to the boil. Cook without stirring until a spoonful of the toffee mixture dropped into a jug of cold water sets into a ball. If you have a sugar thermometer, it should read 138 degrees centigrade.
Remove from the heat, and add the vinegar and bicarbonate of soda to the pan. Take care, as the mixture will bubble up.
Immediately pour the mixture into the lined tin and set aside.
After 15-20 minutes, when the mixture has begun to set, score the toffee into squares with a sharp knife and leave to set completely.
When set, the toffee will break into squares along the lines.