Preheat the slow cooker if necessary; see the manufacturer's instructions. Put all the ingredients in the slow cooker pot, in the order listed. Cover with the lid and cook on high for 4–5 hours, stirring once or twice during cooking. By the end of the cooking time the fruit should be thick and pulpy.
Warm 4 clean jars in the bottom of a low oven. Spoon in the jam, place a waxed disc on top and leave to cool. Seal each jar with a cellophane jam pot cover and an elastic band, label and store for up to 2 months in the refrigerator. (The jam's low sugar content means that it does not keep as long as conventional jam and must be kept in the refrigerator.)