Place apple, lavender sprigs, blueberries and 2 litres (8 cups) water in a pan. Bring to the boil, then simmer over medium-low heat for 45 minutes. Add vinegar and simmer for 5 minutes. Pour into a colander lined with muslin or a clean Chux over a large bowl. Drain overnight.
Discard pulp, then measure strained juice into a saucepan, adding 1 cup of sugar for every cup of juice. Stir over low heat until sugar dissolves, then bring to the boil.
Boil for 25-30 minutes, then test for setting point: pour a little mixture onto a cold plate from the freezer. Freeze for 1 minute, then run your finger through centre of jelly. If it stays in two halves it has reached setting point. If not, continue to boil, testing every 2 minutes. When set, skim any scum from the surface and stir in the lavender leaves.
Pour into sterilised jars and cool completely before sealing. Keeps for up to 3 months.
