Lightly butter a 20cm square baking tin, line with baking parchment and set to one side. Spoon the condensed milk into a large, heavy based saucepan and add the milk chocolate, butter and sugar.
Gently heat, stirring all the time until the chocolate has melted and the mixture is smooth and glossy. This should take around 5 minutes.
Carefully tip the hot mixture into the prepared tin and level off the top. Allow to cool for 30 minutes. Place the Crunchie bars in a food bag and gently bash with a rolling pin into nice chunks. Scatter over the fudge and gently press down so that it stays in place. Chill for an hour or overnight until firm. Cut into 2cm x 4 cm pieces and serve.
Feeling brave? Why not try out some other toppings on top of your delicious chocolate fudge! How about dried fruit, crushed Maltesers or chopped Smarties!