Grease a 20x30cm cake tin then line the bottom and sides with baking paper. Make sure the baking paper's sides are at least 5cm above the sides of the tin. Grease the baking paper with a little butter or oil.
Add sugar, golden syrup, vanilla and water to a deep heavy bottomed saucepan. Bring the mixture to the boil but do not stir. Boil until it reaches the hard crack stage (the temperature on a sugar thermometer reads 149 - 154 degrees C); about 10 minutes. During boiling if there are any sugar crystals on the sides of the saucepan brush the sides of the saucepan with a clean pastry brush dipped in water.
Remove from heat and working quickly. Add the bicarbonate of soda whisking to incorporate. The mixture will bubble when adding the bicarbonate of soda so mind the hot toffee.
Immediately pour into the prepared tin. Let cool and set completely before touching. Break into pieces and serve. Store at room temperature in an airtight container.