Line a 20cm square baking tin with non-stick baking paper.
Put the butter, golden syrup and sugar into a medium pan over a medium-low heat. Heat gently, stirring occasionally, until the sugar has dissolved. Remove from the heat and stir in all the remaining ingredients.
Press the mixture into the tin. Cook for 20 mins until golden brown. Mark the flapjack into 16 squares while still hot. Cool slightly, then turn the flapjack out onto a chopping board. Cut into squares and leave to cool.