De-stalk and wash the blackberries thoroughly and place in your largest pan.
Using a potato masher roughly mash the blackberries.
Add the remaining ingredients to the pan.
Simmer over a medium heat, stir occasionally until the sugar has dissolved.
Bring the jam to a rolling boilfor approx. 10 minutes, stirring all the time.
Reduce the heat to a simmer.
Check that your jam is set by spooning a small amount onto a cold saucer. If it wrinkles when you gently push it, it is set. If not, continue to cook and test again until it's set.
While still hot pour the jam into the hot sterilised jars, place a wax disc on top and seal the jar.
Date and label the jar and store in a cool, dark place.
Once opened refrigerate and use within 3 weeks.