Put two small, sterilised saucers in the freezer – you will use these to test the setting point of the jam. You can sterilise the saucers by washing them in hot soapy liquid, rinsing and placing in an oven at 180C/160C fan/gas 4 for 5 mins. Remove carefully and allow to cool.
Remove the stalks from the fruit, and place the fruit in a large saucepan. Pour in 150ml water, bring to a simmer and cook gently for about 10 mins until the fruit has softened and is starting to break down. Place a sieve over a mixing bowl and push as much of the fruit and pulp through the sieve with the back or a spoon as possible, discarding the seeds and any stalks. You don't need to sieve the fruit if you prefer a seeded jam.
Weigh the liquid and add three quarters of the sugar to the total liquid (for example, if your liquid is 400g, add 300g of sugar). Place back in the pan over a low medium heat and stir to dissolve the sugar.
Once the sugar has fully dissolved, turn the heat up and bring to the boil. Stir occasionally. If using a jam or digital thermometer, wait until the liquid reaches 105C. If not, after about 8 mins, remove a saucer from the freezer and spread a teaspoon of jam over it. Let it sit for 1 min before pushing your finger through the liquid. If it starts to wrinkle, the jam is ready. If not, return to the boil and check again after another minute. Continue to do this until the jam reaches setting point.
Allow to cool slightly, then pour into sterilised jars and allow to cool completely. Store in a cool dark place.