Preheat the slow cooker if necessary; see the manufacturer's instructions. Lightly butter a 1.5 litre (2½ pint) pudding basin and base-line with a circle of nonstick baking paper, checking first it will fit in the slow cooker pot. Sprinkle in a little of the coconut, then tilt and turn the basin until the buttery sides are lightly coated. Spoon half the cherry pie filling into the base of the basin.
Tip the cake mix into a bowl and mix in the oil or egg and water according to the pack instructions. Stir in the remaining coconut, then spoon the mixture into the basin and spread it level. Cover the top with buttered, domed foil and lower the basin into the slow cooker pot.
Pour boiling water into the slow cooker pot to come halfway up the sides of the basin. Cover with the lid and cook on high for 3–3½ hours or until the sponge is well risen, feels dry and springs back when pressed with a fingertip.
Lift the basin out of the slow cooker pot using a tea towel and remove the foil. Loosen the edge of the pudding with a knife, turn out on to a plate and peel off the lining paper. Heat the remaining pie filling in a small saucepan or the microwave until hot. Serve the pudding in bowls with the warm cherries and scoops of vanilla ice cream, if liked.