Melt the butter, chocolate and golden syrup and rum in a heavy-based saucepan. Scoop out about 125ml of this melted mixture and put to one side. Put the biscuits into a freezer bag and then bash them with a rolling pin. You are aiming for both crumbs and pieces of biscuits. Fold the biscuit pieces and crumbs into the melted chocolate mixture in the saucepan, and then add the marshmallows and toasted peanuts. Tip into a 22cm square tin lined with non-stick paper and flatten as best you can with a spatula. Pour the reserved 125ml of melted chocolate mixture over the marshmallow mixture and smooth the top. Refrigerate for about 2 hours or overnight. Cut into 24 fingers and dust with icing sugar before serving.