Put the sugar, syrup, butter and water into a heavy pan, and stir over a low heat until the sugar has melted. Boil steadily until a little of the mixture forms a firm ball when dropped into cold water. Take off the heat and stir in the vinegar and then the bicarbonate of soda. The mixture will froth up in the pan. Pour into an oiled tin and break into pieces when cold. This toffee does not store well and is best eaten quickly.