First sterilize glass or plastic bottles to store the cordial, this is a very important step and can significantly expand the life of a cordial.
Add the blackberries, lemon zest and water to a large saucepan, leave to simmer for 10-15 minutes, the blackberries should be very mushy, make sure you stir regularly.
Then, strain the mixture through a sieve into a jug, making sure you squeeze out all the juice from the pulp, there should be around 500 ml of this liquid.
Return the liquid to a large saucepan and dissolve the sugar over a low heat, once dissolved increase the heat to boiling point and take of the heat before quickly adding in the lemon juice. Pour into the sterlized bottle, serve 1 part cordial to 4-5 part water.