Heat the sugar and butter in a non-stick pan gently, stirring, until melted.

Add the condensed milk and bring to a rapid boil, stirring continuously.

Cook the caramel for a minute or so, or until the filling has thickened.

Put one third of the caramel into a large bowl and mix in the rice cereal.

Press the mixture into the base of the tin and chill until set, about 15 minutes.

Reserve the remaining caramel, then pour it over the base.

Cool, then chill until set.

Place the chocolate into a bowl and microwave gently in short 10-20 second bursts until melted, stirring occasionally.

Spread the melted chocolate over the caramel for the topping, then chill until set.

Remove from the tin and cut into 16 squares, dipping the knife in hot water and drying it on kitchen towel to make it easier to cut through the chocolate.