Place the Chelsea White Sugar, honey, vinegar and butter in a heavy saucepan. Stir until the sugar is dissolved.
Insert a candy thermometer and boil gently until it reaches the hard crack stage (149-154oC).
Remove from the heat and add the bi-carbonate soda, stirring until the mixture swells up.
Pour into a buttered tin (approx. 15cm x 23cm), let cool until half set then mark into squares with an oiled knife. Alternatively, wait until the mixture is cold, turn pan over and knock out with a hammer - it will break into a number of pieces.
Store in an airtight container, packed between single layers of greaseproof wax paper.