Before you start making the jam, put a small plate into the freezer. This will be used for testing the consistency of the jam later on.
Place all of the ingredients into a large saucepan and cook on medium heat, stirring every now and then, until the frozen cherries have defrosted and the sugar has dissolved.
Use a slotted spoon or tongs to remove about a third of the frozen cherries (not the dried cherries) from the mixture and set aside on a chopping board to cool slightly.
Use the handheld blender to purée the jam, making it as smooth as possible.
Strain the mixture through a sieve into a clean saucepan, using the back of a spoon to push as much of the pulp through as possible.
Finely dice the reserved cherries, then add them to the strained jam mixture in the new pan.
Set the pan over medium heat and bring the jam to a rolling boil, stirring gently. Use a metal spoon to remove as much foam as possible from the surface of the jam as it cooks.
Use a thermometer to monitor the temperature of the jam as it boils. Once it reaches between 105-110°C, remove the pan from the heat and grab the plate from the freezer.
Place a small spoonful of jam onto the frozen plate, let it sit for 1 minute, then gently push the jam with your finger. If the jam wrinkles, it's ready. If not, return the plate to the freezer and the pan to the heat. Boil the jam for a further minute, then test again. Repeat this until it passes the 'wrinkle test'.
Once the jam is ready, pour it into a heatproof container. Cover and leave to cool to room temperature before placing into the fridge until ready to use.
The jam will firm up significantly in the fridge. If it is too firm to use when it first comes out of the fridge, just give it a 10 second blast in the microwave to thin it back out a little.