For the blueberry plum syrup, first wash the glass bottles and lids hot and boil them. Let them dry upside down. Place directly where you want to bottle (tray with cloth).
Put water in a wide pot. Wash, halve, seed and cut plums into eighths and add to water. Let the fruit mixture simmer. Stir occasionally.
Meanwhile, sort out and wash the blueberries and add them to the pot as well. Simmer together for about 10 minutes, until both fruits are soft. Stir in cinnamon and lemon juice. Allow fruit mixture to cool in pot.
Then pass through a liquid softener, strain, or blend very finely with a hand blender. If you want no seeds and less pulp in the syrup, strain through a cloth-lined sieve into a saucepan. Add sugar.
Cook until syrupy, stirring occasionally, about 16 minutes. Immediately pour the blueberry plum syrup into bottles, to just below the rim, and seal. Turn upside down and allow to cool. Store in a dark and cool place.