Put the butter, sugar, syrup and water into a large saucepan with a heavy base and stir over gentle heat until the sugar has dissolved. Boil steadily to 280°F., or until mixture forms a very firm ball when you drop a little into cold water. Remove from heat, stir in the vinegar and then the bicarbonate of soda. The mixture will rise considerably in the pan, giving the ‘honeycomb’ texture. Pour into an oiled tin. When cold, break into pieces.