Preheat the oven to 170°c / Gas 3. Line a 8 inch square tine with greaseproof paper.
Melt the butter, sugar and golden syrup in a large saucepan over a low heat, stirring occasionally. Stir in the oats and flour until well combined. If using sultanas, these can be added now.
Spoon the flapjack mixture into a baking tin and spread level. Bake for around 25-30mins or until light golden.
Cut into squares will still hot. Leave to cool for 5mins. Remove from the tin, separate and transfer to a wire rack to cool complete. Store in an airtight tin.