Found this recipe on http://www.fiddes.net 

Pick blackberries, clean and weigh and put in a pan

Add half the weight of the blackberries in apples (I cant buy cooking apples in France, but I use the most acidic possible) There is no need to core, or peel the apples; just wash and chop roughly, then add to the pan

Add water to nearly cover the fruit and then simmer until well broken up

weigh a bowl large enough to contain the fruit pulp

Sieve the fruit pulp through a nylon seive, into the bowl; discard anything which will not go through the sieve.

Weigh the bowl+pulp and calculate the weight of the pulp by subtracting the weight of the bowl from the total

Return pulp to the pan and add the same weight of sugar

Boil until setting point is reached, then pour into hot sterilised pots.