Line a 20 x 30 cm (8 x 12 inch) baking tin with baking paper. Melt the chocolate, golden syrup and butter in a large saucepan over low heat (do not cook; you want the butter to be just melted). Add the biscuit chunks and stir until well coated. Pour into the baking tin and use the back of a spoon to flatten the mixture. Cover with baking paper and refrigerate for 1 hour, or until set. Turn out onto a clean chopping board and, using a serrated knife, cut into bar shapes.