Wash the crab apples and cut off any bad bits. Cut into chunks or slices (no need to peel or core). Place in a large stainless steel pot and add the water.
Bring to the boil, then reduce the heat for about 30 minutes until mushy.
Line a colander with a muslin cloth or jelly bag and set it over a large bowl. Spoon in the apple mixture without pressing down and allow to stand for at least 4 hours or overnight until all the juices have dripped out. Do not squeeze it, as this will make the jelly cloudy.
Measure the apple juice into a clean stainless steel saucepan, and for each 250ml/1 cup juice, add 200g/1cup sugar. Add the lemon juice. Heat gently until the sugar has dissolved, then increase the heat to boiling point. Continue to boil until the mixture reaches 105°-110°c (220-225°f). To test to see if it's ready take a cold plate from the fridge and add a small amount, and wait for a few minutes. It should wrinkle with your fingertips when ready.
Pour into hot sterilised jars. Seal immediately and allow to cool.
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Notes
Crab Apple jelly doesn't take as long to set as it would to make jam because there is a lot of pectin in apples.