Roughly chop the butter and break the chocolate into squares. Gently melt the butter, chocolate and syrup in a heavy based saucepan, stirring to ensure everything is well mixed. Scoop out 2-3 tablespoons of this melted mixture and put aside on a warm saucer or small dish.
Place the biscuits and Maltesers (or Crunchie) into a freezer bag and bash them with a rolling pin. You are looking for both crumbs and pieces of cookies. Fold theses broken biscuit pieces and crumbs into the melted chocolate mixture. Cut the marshmallow into small pieces and stir into the mixture too.
Pour the mixture into a 9" (23cm) square foil tray or cake tin and flatten with a spatula. Pour over the reserved melted chocolate mixture and either smooth the top or create patterns in it with the back of a fork..
Refrigerate for at least 2 hours (overnight is better). Cut into squares to serve.