Put the sugar and lemon juice in a small pan and heat gently to dissolve the sugar. Simmer for 2 minutes to make a syrup. Remove from the heat and leave to cool. This can be made in advance and kept in the fridge for up to a week.
To serve, pour the lemon syrup into 6 champagne flutes or tall glasses, top up each glass with the chilled sparkling wine, then stir quickly to mix before serving with mint sprigs tucked in the top.