Prepare the cherries - wash, pit and chop half of them. Leaving the other half as whole cherries gives a chunkier texture but chop them all for a smoother textured jam.
Soften the fruit - add cherries to your preserving pan along with 3 tbs water and cover with a lid. Cook on a gentle heat for 10 to 20 minutes. Shake the pan a couple of times to prevent fruit sticking to the pan. Remove lid and cook on a gentle heat for 5 minutes.
Add sugar - stir in jam sugar and keep stirring until fully dissolved - no visible grains should remain. Increase the heat to medium.
Add lemon - stirring in lemon juice once sugar is dissolved avoids interfering with pectin setting.
Boil to setting point - bring the mixture to a gentle rolling boil and cook for 5 to 10 minutes until it reaches 104°C / 220°F.
Check the set - remove your pan from the heat. Test for setting point by dropping a teaspoon full on a chilled saucer. Return to the fridge for 1 minute. Push the jam gently with your fingertip - if it crinkles, it's ready. If not, reboil for a couple of minutes and repeat until setting point is reached.
Pot the jam - let the jam rest for 10 minutes (to prevent fruit from floating), then ladle into warm, sterilised jars. Fill to within 6mm of the top, remove air bubbles, and seal tightly.
Store in a cool, dry place out of the sunlight and use within 12 months.