Place 1 kg of the blackberries in a large jam pan with the water and slowly bring to the boil, softening the fruits for about 10 minutes.
Add the jam sugar and lemon juice and bring to the boil, stirring to dissolve the sugar, then add the remainder of the blackberries.
Boil rapidly, stirring constantly, for 15-20 minutes.Now, use the wrinkle test to check for setting point. When its ready take off the heat and skim off any scum from the surface.
Stir through the gin and leave for a few minutes so that the alcohol burns off. Ladle into warm dry sterilised jars and seal. It will keep unopened for 6 months. Once opened refrigerate and eat within 4 weeks.