Place the flour, cream of tartar, and sugar in a large bowl. Mix.
Cut the butter into cubes and rub it gently into the flour and sugar mixture. Don't overmix. You want to leave a rough, not fine, texture.
Add approx 170 g of the blueberries then pour in the milk.
Stir together then tip onto a floured surface and form into a round.
Lift the scone round onto a large baking case.
Place a second large baking case in the slow cooker pot. Note: I used my round 3.5 litre slow cooker for this. If you have a bigger slow cooker you might need to double your quantities or use a cake tin inside the pot.
Put the case with the scone into the pot inside the other case. This adds an extra layer to prevent burning at the sides.
Cook on high for one and a half to two hours, until cooked through and golden around the sides. Use a tea/dish towel or squares of kitchen roll under the lid to prevent moisture dripping onto the scone.
Use the case to pull the scone out of the pot and leave to cool.
Whip the double cream, I like to do this by hand using a whisk but you can also use an electric whisk.
Split the scone in half, spread the whipped cream using a spatula knife and then top with the remaining blueberries. Replace the top of the scone and serve!