Fill your sink about an inch or so deep with ice cold water.
Line a 30x30cm tin (or similar size) with greaseproof paper.
Take a large, high-sided saucepan and place the sugar and syrup in it. Melt slowly together over a low-ish heat with your sugar thermometer in the mixture without stirring. If you need to you can swirl the mixture around to incorporate all the sugar.
Heat to ‘hard crack’ temperature (see below), place the bottom of the saucepan into your sink to stop the cooking and then add the bicarb, stirring with a wooden spoon. The mixture will bubble up furiously – after a few seconds pour it into the tray and leave to cool.
Once cooled, crack into pieces and serve.
Remember – when cooking with caramel, take extra care – hard crack stage is 300°F!