Cook sugar, milk, salt and corn syrup in 2 litre saucepan over medium heat, stirring constantly, until sugar is dissolved. Cook, stirring occasionally, to 120ºC on candy thermometer or until small amount of mixture dropped into very cold water forms soft ball that flattens when removed from water. Remove from heat. Add margarine. Cool mixture to 50ºC without stirring. (Bottom of pan will be lukewarm.) Add cardamom. Beat vigorously and continuously until candy is thick and no longer glossy, 5 to 10 minutes. (Mixture will hold its shape when dropped from spoon.) Quickly stir in nuts. Spread mixture in buttered 9x5x3-inch loaf pan. Let stand until firm. Cut into 3 cm squares.