Place the sugar in a heatproof bowl and pop in a moderate oven for 10 minutes to heat up. You can also place the jars into the oven to warm, to prevent them cracking when the hot jam is poured into them.
Put a saucer in the freezer for testing the jam later on.
Place the fruit (which can be frozen) in a saucepan with the lemon juice and heat up. Simmer for 3 minutes and crush most of the fruit with a potato masher. Add the warm sugar, stir to dissolve and bring up to the boil. Boil for 3-4 minutes over a high heat, stirring regularly.
To test to see if the jam is cooked, take a spoonful of the jam, place it on the frozen plate and allow it to sit for a few seconds. Then push your finger through the blob of jam - if the skin on top forms a wrinkle when pushed, it is cooked. Remove from the heat immediately and pour into sterilised jars or a bowl. A jam funnel is handy for this if you have one. Place the lids on top. The jam will set as it cools.