Pour the heavy cream into the inner pot of your slow cooker. Set the temperate to the slow cookers lowest temperature, usually "warm" or "low." It should be cooking at around 180°F (82°C). If you cook it much higher than this the cream will burn and you wont get the desired result.
Cover and cook for 8-10 hours. When it is done it will have developed a film at the top and become a darker cream color and be slightly brown around the edges. It will still be pretty thin when it is done cooking, it gets thick while cooling.
Gently remove the inner pot from the slow cooker, trying not to allow too much movement of the cream or the liquid part will break through and you will get less clotted cream. Let the pot cool to room temperature, and then gently place the pot into the refrigerator to cool for 6-8 hours.
Once the top layer of cream is solid use a spoon to gently pull up the thick layer of clotted cream at the top and place this in a separate container. There will still be liquid in the bottom of the pot, and you can use that later as cream to make scones or for anything else you need cream in. Gently stir the container of clotted cream to make it creamy. Store it in an airtight container in the fridge for up to 5 days, but warm it to room temperature before using it for the best flavor and texture.