1.Blend or process raspberries and juice until smooth.
2.Combine water and caster sugar in medium saucepan, stir over heat, without boiling, until sugar dissolves; bring to a boil. Reduce heat; simmer, uncovered, without stirring, 5 minutes.
3.Add raspberry mixture to pan; bring to a boil. Reduce heat; simmer, uncovered, 2 minutes. Strain raspberry cordial into medium jug, discard seeds; refrigerate 30 minutes.
4.Mix one part cordial to three parts chilled water or soda water.