Wash the apples. Cut out and discard any bad bits, then cut into thick slices. There is no need to peel or core the apples. Place in a preserving pan or large saucepan and add the water.
Bring to the boil, then reduce the heat and cook for about 30 minutes until the apples are very soft and mushy.
Line a colander with a square of muslin (or use a jelly bag and stand) and set it over a large bowl. Spoon in the apples and juices and allow to stand for at least 4 hours or overnight until all the juices have dripped out. Do not be tempted to squeeze it to hurry it along as it will make the jelly cloudy if you do.
Measure the apple juice into a large saucepan or preserving pan and for each 250ml/9floz (1cup) juice add 200g/7oz (1 cup) sugar. Add the lemon juice. Heat gently stirring until the sugar has dissolved, then increase the heat and bring to a rolling boil.
Continue to boil until the mixture reaches 105-110℃ /220-225°F on a thermometer and test for set. (A small blob will wrinkle when placed on a cold plate and left for a few minutes).
When the jelly has reached setting point, pour into warm sterilised jars. Seal immediately and allow to cool.
Label when cold and store in a cool, dark place. The jelly will keep for up to 1 year.