Heat the oven to 180C (160C fan, gas mark 4). Grease and line a 20cm square tin with baking paper.
Put the butter, sugar and golden syrup in a medium saucepan and warm over a low heat, stirring, until the butter has melted and the sugar has dissolved. Do not let it boil.
Take the pan off the heat and stir in the oats and salt until every oat is coated.
Tip the mixture into the tin and press down firmly with the back of a spoon, especially into the corners.
Bake on the middle shelf for 20 to 25 minutes, until the edges are golden brown and the centre is just set.
Cool in the tin for 10 minutes, then mark into 12 squares. Leave until completely cold before lifting out and cutting.