1. Wash and cut up the apples without peeling or coring. If windfalls are used, weigh after removing the damaged parts.
2. Pick over and rinse the blackberries if necessary.
3. Simmer the fruits separately, giving the larger share of water to the apples.
4. When the fruits are tender, after about 1 hour, mash them and pour both into a scalded jelly bag.
5. Leave to drip for 1 hour, then measure the extract, return it to the clean pan, and heat gently.
6. Add 800g sugar for each litre of extract.
7. Stir over low heat until dissolved.
8. Boil steadily until setting point is reached.
9. Remove from the heat, skim, pot, cover, and label.