Wash the blackberries in a colander and remove any stems that remain on the fruit.
Wash the apples and cut them out roughly without peeling.
Put the blackberries and apples in the pan with the waterand simmer until the fruit is soft.
Hang your jelly bag over a large bowl, and ladle the fruit mixture and juice into the jelly bag.
Leave to strain overnight.
Measure the juice and weigh the sugar. You will need 450 g of sugar for every 550 ml of juice.
Put the juice and the sugar into a large pan and heat gently untill all the sugar has dissoved.
Bring to a rolling boil and boil for 15 minutes. Remove from the heat before testing for set using the wrinkle test.
If not set bring back to a boil and boil for a further 5 minutes before testing again.
Skim the bubbles from the top of jelly and with a large spoon OR add a tsp of butter which will disperse the froth.
Fill the jam into sterilised jars using a jam funnel or carefully ladling the hot jelly into the jars and seal.