Mix together the first 180 g of chocolate and golden syrup. Melt together gently. Place the cornflakes in a large bowl and pour the melted chocolate/syrup over them. Mix to combine thoroughtly.
Spread the cornflakes in the base of a large loaf tin (lined). Press down well to form a solid base layer. Chill in fridge until set.
Gently heat the peanut butter in a bowl (I used the microwave, 1 minute at 600 W) and stir in the icing sugar. Pour it over the chilled chocolate cornflake mixture and spread evenly to form the second layer. Chill in fridge.
Melt the second lot of chocolate and pour over the chilled peanut butter/cornflake slice. Spread evenly. Chill in fridge until set. Cut into slices to serve.