Before you start, sterilise your jars (see tip below) and put a plate in the freezer to chill.
Tip half the raspberries into a preserving pan and add the lemon juice.
Mash the berries to a pulp over the heat with a potato masher, then leave to cook for 5 mins.
Tip the cooked berries into a sieve over a bowl, then once all of the juice has drained off, firmly work the pulp through the sieve with a wooden spoon until you are left with just the seeds.
Tip the juice and pulp back into the preserving pan and stir in the sugar.
Heat gently, then add the remaining whole raspberries.
Bring to the boil, then boil rapidly for 5 mins.
Remove from the heat and drop a little jam onto the chilled plate.
Now push your finger through it – it should wrinkle and look like jam. If it doesn’t, boil for 2 mins, then test again.
The top of the jam may look like it has sediment on it, but I find that if you stir it well as it cools, a little of this disappears.
Pour into the jars and seal.
It will keep unopened for a year, although the lovely bright colour will darken a little.