Put everything into a very large, heavy-bottomed saucepan and place over a high heat.
Bring to boil stirring constantly. Mixture will start to burn sugars, and change colour from white to light brown. Remove from heat and whisk for 2 minutes, trapping air will inflate mixture, don't panic, start to stir again until mixture starts to cool and stops inflating (can stand pot in sink full of cold water to speed cooling).
Pour the mixture into a large baking tray lined with greaseproof paper. Leave for about 40 minutes, then use a sharp knife to mark into small cubes. Allow to set fully then break into cubes and serve.