In a small pot over medium heat, bring to a boil the 1/2 cup of blackberries, the sugar and the water. Let simmer for 5 minutes. Strain the syrup through a sieve and refrigerate for 1 hour or until completely chilled. Half-fill two large 1 3/4 cup (430 ml) wine glasses with ice and a few blackberries. Divide the syrup between the two glasses, add the Prosecco and the Pimm’s No. 1 and stir with a spoon.