Wash blackberries in cold water and remove any stalks. Drain well.
Put 100ml water and the lemon juice in a large pan then put in all the blackberries. Do not add the sugar yet!
Put the pan on a medium heat to slowly raise the temperature, do not use a high heat otherwise the bottom might burn. Start to press the blackberries down with the back of a spoon to create some juice, then cover the pan with a lid. Stir the mix occasionally and press the blackberries down again to help create a juicy pulp. Once simmering, continue to heat on a medium setting with the lid on for about 1 hour, till all the fruit has broken down to a soft pulp and there's lots of juice.
Remove from the heat and add the sugar a bit at a time, stirring continuously till dissolved. Return to the hob and increase heat to high. It is now important to continue stirring constantly while the mix is heating up to boiling point, this will ensure the sugar does not burn on the bottom of the pan during heating. Once the mix is boiling, leave the heat on high with the lid off to achieve a rolling boil state (over 105 degrees C); this will start the setting process using the fruit's natural pectin content. Boil the mix with the lid off, stirring frequently for a further 50 minutes, the last 10 minutes with the heat on maximum to achieve a vigorous rolling boil if you can.
Test for the setting point by dropping a little jam on a cold plate. If you push it with your finger and it wrinkles, it is ready.