450g granulated sugar
Preheat the oven to 170°C, gas mark 3.
To sterilise the jars, place 3 thoroughly clean 200ml jars on their sides in the oven for 10 minutes. Then turn off the oven, leaving the jars inside until the jam is ready to pot. Place 2-3 saucers in the freezer to chill.
Place berries in a pan and cook over a gentle heat for 2-3 minutes until the juices are just beginning to run. Add the sugar and stir over a gentle heat for 1-2 minutes until the sugar has dissolved. Then increase the heat and bring to a vigorous boil for 5-10 minutes.
Remove from the heat and test the jam by dabbing a little on one of the cold saucers. Cool for a few seconds, then push the jam with your fingertip. If it wrinkles, it has reached setting point. If not, boil for a further 2 minutes then test again. When the setting point is reached, spoon the jam into the jars. Cover the surface with a disc of waxed paper and seal with a lid.