Combine ingredients in a large heavy-based saucepan and bring to the boil, stirring occasionally, over low heat. Cook, stirring occasionally, until mixture reaches 160C on a sugar thermometer.
Line a 20cm-square cake pan with baking paper. Pour mixture into pan and smooth top. Allow to cool slightly, then loosen edges from pan and, using a sharp knife, cut into diamonds. Place on baking paper to cool completely. Brittle will keep in an airtight container for up to two weeks.