Place all ingredients, except sugar, and 1 cup water in a large saucepan, bring to the boil over high heat, reduce heat to medium and simmer for 15 minutes or until soft. Add sugar, simmer for another 10 minutes, then discard star anise.
Process plum mixture in a food processor until puréed, then strain through a fine sieve into a clean saucepan. Cook over medium-high heat for 5 minutes or until slightly thickened. Pour hot sauce into warm sterilised bottles and seal immediately. Cool and store in a cool, dark place for at least 2 weeks before opening to allow flavours to develop. Unopened sauce will keep for up to 3 months.