Butter the sides of a heavy-based saucepan. Add 350ml water with the sugar, salt, syrup and vinegar. Bring to the boil, stirring, until the sugar has dissolved. Cook, without stirring, until it reaches hard-ball stage.
Pour the syrup slowly over the warm popcorn, mixing well. With lightly buttered hands, form the mixture into eight 5cm balls. Allow to dry thoroughly and wrap the balls in coloured cellophane and tie with ribbon to make small gifts.