Preheat the oven to gas mark 4. Line a 20x30cm brownie tin with greaseproof paper or baking parchment. Stand the golden syrup in a pan of boiling water to to make it easier to pour.
In a large saucepan, heat the sugar and margarine until melted and stir to combine well. Add the golden syrup and oats and stir again. Spoon into the already prepared tin. Place in the oven on the middle shelf and cook for 25 minutes.
Leave the flapjacks in the tin for 10 minutes and then cut into 25 pieces and leave to cool completely in the tin before transferring into an air tight tin. They will keep well for up to 3 days.
Melt 25g of milk or dark chocolate (according to preference) and drizzle over the cooled flapjacks before you have cut them.