Place to cherries, sugar and tarragon in a small pan with 1
1/2 cups of water. Gently bring to a boil, stirring until the sugar
dissolves, then simmer for 10 minutes until the cherries are very soft.
Remove from the heat, then pass through a sieve to remove the stones and
tarragon and puree the cherries.
Stir in the lime juice, then chill until ready to serve.
Transfer to a jug and top up with soda water. Pour into ice filled glasses
and serve garnished with lime twists.