To make the blackberry syrup, heat the blackberries, sugar and measured water in a saucepan until boiling. Reduce the heat and simmer for 20 minutes
Remove from the heat, add the lemon zest and thyme and leave to steep for 20 minutes. Strain and pour into a sterilised glass bottle. This will keep in the sealed bottle in a cool, dark place for up to 2 weeks
To make the cocktail, put the gin, syrup and lime wheel into a cocktail shaker. Shake well and muddle.
Add ice cubes and shake, then strain into a chilled goblet over ice. Top up with the tonic water and garnish with a rosemary sprig