1. Mix together all the ingredients in a large heavy based saucepan and turn on the heat. Gently warm until the butter melts.
2. Pop the thermometer in the pan and turn up the heat to a steady boil. Stir it often and allow the temperature to reach 116°C (soft ball stage). Don't be tempted to take it off the heat too early as the fudge won't set.
3. Turn the heat off and allow to cool to 110°C. In the meantime line a 20cm x 20cm baking tin with parchment paper.
4. Once the fudge has cooled, beat the mixture with a wooden spoon until it reaches 60°C. It takes a while but don't give up otherwise your fudge will form large sugar crystals and have a grainy texture.
5. Beat for a few more minutes (and stir in any nuts and raisins if using) and pour into the lined tin. Smooth the top and leave to set at room temperature overnight.
6. Once set, cut into your favourite shapes before serving. Store in an airtight container for upto 4 days.