Line a roasting tin with greaseproof paper. Slowly heat the caster sugar and syrup slowly over a medium heat until the sugar has dissolved. Bring to the boil then turn heat down and simmer until the mixture has just started to turn a lovely amber colour and your thermometer reads 150 degrees. (I use a meat thermometer rather than a sugar one and it seems to work just fine).

Now take the saucepan off the heat and quickly stir in the bicarbonate of soda. Once it has started to foam up tip it into the roasting tin; once you've poured it out it is very important to not touch it or the bubbles will collapse. Leave for an hour until it has gone completely cold then break into chunks. I keep intending to dip the chunks in melted chocolate to make home-made Crunchie bars but so far it hasn't lasted long enough!