Pour the double cream into your slow cooker and put it on the lowest setting. On mine this is warm (as compared to low or high). It's roughly 70C.
Leave the lid off to allow air exposure and leave for 3 - 4 hours.
Switch the heat off and leave for another hour.
Move the pot of cream very gently to the fridge making sure not to break the surface or shake it in any way.
Keep in the fridge overnight, then scoop the top thick layer off carefully, putting that in an airtight container and use the remainder as buttermilk in bread or scones which you can then top with your clotted cream!
Recipe by Keeper of the Kitchen at https://www.keeperofthekitchen.com/2015/08/19/diy-clotted-cream-in-the-slowcooker-or-crockpot/