Melt together the sugar, butter and golden syrup in a saucepan over a medium heat, stirring occasionally. Place the oats in a large mixing bowl and add the melted ingredients to them. Stir the wet ingredients into the oats, working quickly to combine as thoroughly as possible.
Pour into a 20cm square tin and level with the back of a spoon. Leave to set until cool. Turn out onto a chopping board once cool and cut into rectangles. Place the chocolate in a heatproof bowl.
Sit over a saucepan of gently simmering water and stir occasionally until the chocolate has melted. Pour the chocolate on top of the rectangles of oat bars and serve immediately.