Grease and line the base and sides of a 1 pound loaf tin pan with baking parchment. Break the fruit and nut chocolate into pieces and place in a heatproof bowl with butte. Place the bowl over a pan of barely simmering water and stir the chocolate gently until it is melted and smooth. Remove the bowl from the pan and leave to cool for 20 minutes.
Break the crackers into small pieces. Finely chop the white chocolate. Stir the biscuits and white chocolate into the melted mixture until evenly combined. Turn the mixture into the prepared tin and pack down gently. Chill for about 2 hours or until set. Cut the mixture into slices.