Place blackcurrants (no need to strip them from the stems) in a heavy based pan with just enough water to cover the base of the pan. Heat gently to release the juice then continue to simmer until the fruit breaks down. Strain through a jelly bag or muslin then add sugar. Add 10g of citric acid for every litre of liquid. Reheat gently to about 85°C until all sugar is dissolved. Bottle hot to preserve. To use, dilute to taste.