1) Prepare four pans, each 40-by-25-by-2.5cm, by buttering them well. Set aside.
2) In a nonstick saucepan and stirring constantly, heat the sugar, golden syrup, water and salt over medium heat until the sugar has dissolved.
3) Over a high heat and using a candy thermometer, cook the sugar mixture to a hard crack stage (143C).
4) Add the nuts and butter and cook to 150C, stirring all the time to keep the nuts from burning. Pull off the heat at 150C and stir in the bicarbonate of soda while beating to a froth for 30 seconds.
5) Pour the mixture at once onto the four pans. Spread with a spatula as thin as possible. As the brittle cools you may use gloves to hand stretch the brittle, which will give it a better eating quality.
Note: This brittle has a great shelf life; if kept in airtight zip bags or containers, it will keep for 2 months.