Heat the oven to 180C/fan 160C/gas 4. Scatter the nuts onto a baking sheet and toast in the oven for 5-10 minutes, tossing regularly until brown, then spread evenly on a piece of baking paper on a large baking tray along with the pretzels.
Gently heat the sugar, golden syrup, butter and 4 tbsp of water in a large pan until melted. Turn up the heat until the mixture is bubbling, then cook until it has turned a rich golden brown – this will take about 10-15 minutes, depending on the heat
Keep a close eye on the pan and give it a stir every now and then. Once the mixture is golden brown, remove from the heat and pour straight over the nuts and pretzels.
Sprinkle generously with sea salt flakes. Leave to set before breaking into shards and packing up. The brittle will keep for 1 week.