Add the blueberries and measurement water to a preserving pan and cook gently for 10 minutes, until softened, crushing from time to time with a wooden spoon or potato masher.
Add the sugar (no need to warm such a small quantity first), honey and lemon juice and heat gently, stirring from time to time, until dissolved. Bring to the boil, then boil rapidly until setting point is reached (10–15 minutes).
Skim with a draining spoon or stir in butter, if needed. Ladle into warm, dry jars, filling to the very top. Cover with screw-top lids, or with waxed discs and cellophane tops secured with elastic bands. Label and leave to cool.
To serve, a spoonful of this jam and a dollop of whipped cream makes a tasty addition to porridge.