Grease a swiss roll tin with a knob of butter; set aside.
In a heavy saucepan over low heat, combine sugar, golden syrup and vinegar. Cook until sugar is dissolved, stirring frequently. Increase heat to medium; cook until a sugar thermometer reads 118 degrees C (firm-ball stage), stirring occasionally. Add treacle and 30g of butter. Cook, uncovered, until a sugar thermometer reads 126 degrees C (hard-ball stage), stirring occasionally. Remove from the heat. Add bicarb and mix well.
Carefully pour the mixture into the prepared tin. Let sit until hardened. Break off bits of toffee and store in an airtight container.