Peel and core the apples. (If grating leave the skin on if desired.) Add with the blueberries and lemon juice to a heavy-based saucepan.
Cook gently over a low heat until the fruit is soft and cooked.
Turn the heat up and bring the mix to a rolling boil.
Add the sugar gradually, stirring until the sugar is dissolved.
Boil rapidly for 10 minutes, then test for setting. If necessary boil for another 2-3 minutes, then test again.
Take off the heat and let settle for a few minutes - remove any scum from the side of the saucepan.
Pour into hot, dry, sterilised jars and seal.
Store in a cool dry cupboard.