Place the blackberries in a glass or china bowl and pour the vinegar over. Leave to stand for at least 24 hours, stirring and pressing the fruit regularly, to extract the juices. Strain the liquid into a large saucepan or small preserving pan and bring to the boil. Add the sugar, stirring until it is all dissolved, then add the honey, stirring well. Bring back to the boil and boil hard for 5 minutes. Allow to cool completely. Originally this syrup was bottled and stored, one tablespoon being added to a glass of hot water to form a bedtime drink. However, the syrup can easily be frozen in ice-cube trays and then stored in bags in the freezer to be used when required. A pleasant nightcap that is also said to be good for reliving a cold.