Begin by placing the blackcurrants and sugar in a deep rectangular dish. Crush the berries with the back of a spoon or a potato masher.
Place the blackcurrant and lemon balm leaves in a pan with 100ml water, bring to a simmer and then remove from the heat to infuse. Leave to cool. Add the infusion to the blackcurrant mixture and stir. Cover and place in the fridge for 3 days. Stir a couple of times within that period.
When it's ready the blackcurrants will be sitting in a rich purple syrup. Now pass the the crushed berries through a fine sieve. Add the vinegar to the berry syrup and stir with a sterilised spoon. Pour into the sterilised bottle and seal. Leave undisturbed for at least 3 days before drinking. The shrub mellows as it matures.
Serve diluted with sparkling water or tonic. Keep in the fridge for up to 3 months.