Preheat your oven to 100℃ fan / 120℃ normal. Put 2 small plates in the freezer (to test when the jam is ready) and, once it is up to temperature, put a freshly-washed jam jar (at least 370ml in size) in the oven for 15 minutes to sterilise it.
Place the cherries in a heavy-based pan with the sugar, star anise and lemon juice. Cook down over a low-medium heat, stirring regularly until the cherries have defrosted and the sugar has melted - it is important to go low and slow at this point so that the sugar does not caramelise. Turn up the heat to medium-high and simmer for 20 to 25 minutes.
Remove the jam from the heat. Fetch a plate from the freezer and put a small blob of jam on it. Leave for 1 minute and then gently prod it with your finger - if the surface has formed a skin that wrinkles when pressed, the jam is ready. If not, keep simmering for another couple of minutes and try again with the other cold plate.
Once ready, remove and discard the star anise and let the jam sit for 5 minutes - your jam jar should also be out of the oven for at least 5 minutes. Pour the jam carefully (a metal funnel is useful) into the jam jar and screw on the lid immediately. Unopened, the jam will last for up to 12 months in the cupboard and, once opened, up to 1 month in the fridge.