Heat the oven to 180C/160C fan and line a 20cm square tin with baking paper.
Put the butter, golden syrup and sugar in a saucepan over a low heat and stir until everything has melted together into a smooth glossy liquid. Do not let it boil.
Take the pan off the heat, add the oats and salt and stir until every single oat is coated.
Tip the mixture into the tin and press it down firmly with the back of a spoon. The harder you pack it in, the better the flapjacks hold together later.
Bake for 18-20 minutes until golden at the edges but still slightly soft in the middle.
Mark into 12 pieces with a knife while still warm, then leave to cool completely in the tin before lifting out and cutting cleanly.