Wash the damsons and place them into a large baking dish along with the brown sugar. Place them in an oven pre-heated to 180°C and roast for 40 minutes.
Remove from the oven and set aside until cool enough to handle.
Pass the damsons through a sieve into a large preserving pan. Pick through the pulp to remove all the stones, placing the stones in a large jug. Add the remaining stoneless pulp into the preserving pan.
Fill the jug filled with stones with the 600ml water. Swirl around so all possible damson juice is loosened from the stones.
Pour the water through a sieve into the preserving pan. Finally, discard the stones.
Zest the oranges, setting aside the zest until the very end. Then juice the oranges. Add the orange juice to the preserving pan, along with all the spices.
Bring to a boil and add the granulated sugar, stirring to dissolve.
Once the sugar has dissolved add the lemon juice. Bring to a rolling boil.
Boil the jam for about 10 minutes until the temperature reaches 104°C or passes the saucer wrinkle test*.
Remove from the heat, stir in the orange zest and decant into sterilised jars*.