Bring the caster sugar and water to the boil, then add 8 sprigs of thyme and leave to cool. Peel, stone and chop 3 plums and muddle with 6 tablespoons of the thyme syrup and the lemon juice to release juice from the fruit. Fill four glasses a third full with crushed ice and divide the fruit & syrup mixture between them equally. Top each with prosecco and serve with more fruit slices and thyme sprigs.
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