In a medium saucepan over medium high heat, combine the caster sugar, dark brown soft sugar and golden syrup. Bring the mixture to a rapid boil, then remove from heat. Stir in the peanut butter until well blended. In a large bowl, stir together the cornflakes and peanuts. Stir the peanut butter mixture into the cornflake mixture and mix until evenly distributed. Press the mixture into a greased 20x30cm or similar sized tin.
In the microwave or in a metal bowl over a pan of simmering water, melt chocolate, stirring frequently until smooth. Remove from heat and spread over the tops of the bars. Let cool before cutting into bars.