In a large saucepan slowly melt together all the ingredients except the baking soda. Do not stir. Boil until a drop hardens in cold water (240°F on a sugar thermometer). Stir in the baking soda. The toffee will immediately foam up as the vinegar releases the gas from the baking soda.
Pour out onto a greased slab and while just cool enough to handle fold the edges towards the centre and pull repeatedly until the whole is a pale yellow colour.
Allow to cool and harden in a greased tin and break into chunks with a hammer.