Place water, sugar, and cranberries into a saucepan over medium-high heat and bring to the boil.

Immediately reduce to low heat and simmer for 5 minutes, stirring until the sugar has dissolved.

Strain the syrup into a bowl and discard the solids. Leave to cool.

To make the cocktail, slice the limes in half and squeeze into a large pitcher.

Add the tonic water and vodka.

Add the cooled syrup to the pitcher and stir to combine.

Pour into glasses filled with ice and top with fresh cranberries and mint.