1. Put the fruit into a preserving pan or large heavy-based saucepan, add 50ml of water and 1 1/2 tbsp of lemon juice and bring to the boil.
2. Lower the heat, simmer the blackberries for 15 minutes until the fruit has softened.
3. Tip in the sugar, stir over a low heat until the sugar has completely dissolved.
4. Raise the heat, bring to a full rolling boil then rapidly boil the blackberries for 10 – 12 minutes, avoid stirring until the setting point of 105 oC is reached.
5. Remove from the heat; skim off any excess foam, then stir a knob of butter across the surface to help dissolve any remaining foam.
6. Leave for about 15 minutes so the fruit can settle.
7. Pour into sterilised jars, label and seal.