Add the blueberries and water to a small saucepan and cook on a medium-high heat for 4-5 minutes, squashing the berries with the back of a spoon to release the juices.

Strain the mixture through a sieve and leave to cool.

In a cocktail shaker, add the blueberry mixture, vodka and sweetening syrup. Shake well to mix.

Add ice to a glass, pour over the cocktail, and top up with soda water as desired.