Heat the milk chocolate gently in a heavy-based pan with the evaporated milk, stirring frequently, until the chocolate has melted. Remove from the heat and transfer to a bowl. Leave until cool, but not set. Stir in the biscuits, dried fruit and nuts.
Grease the base and 3 sides of a deep 18 cm (7 inch) square cake tin and line with clingfilm. Prop up one side of the tin so that it sits at an angle of 45 degrees and the unlined side of the tin is uppermost. Spoon in the cake mixture and level the surface. Leave until firm, then transfer to the refrigerator to set completely. Remove the cake from the tin and peel away the film.
Melt the plain dark chocolate (see