Heat the oven to 180C/350F/gas mark 4. Line a baking tin with non-stick baking paper.
Heat the butter, sugar and golden syrup in a pan until just melted.
Add the muesli and mix well. Tip into the baking tin and spread evenly.
Bake for 15-20 minutes until golden. Mark into 12 bars and leave to cool completely in the tin, as it falls apart when warm. Cut when cold.