Add the peaches, sugar and lemon juice to a preserving pan. Slit the vanilla pod lengthways, then scrape out the seeds and add to the pan. Cut the pod into very thin strips, and add this to the pan. Cook very gently, uncovered, for 20 minutes, until the peaches are tender and the sugar has dissolved to make a syrup, stirring from time to time.
Bring to the boil, then boil rapidly until setting point is reached (5–10 minutes). Skim with a draining spoon or stir in butter if needed.
Leave the conserve to stand for 5–10 minutes so that the fruit will not rise in the jars, then 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.