To make the crepe batter, place the flour in a mixing bowl and make a well in the center. Add a little of the milk with the egg and the oil. Whisk flour into the liquid, then gradually whisk in the rest of the milk, keeping the batter smooth. Cover the batter and put in the refrigerator while you prepare the filling.
Quarter, peel and core the apples. Slice them into a pan and add the black currants and water. Cook over gentle heat for 10-15 minutes, until the fruit is soft. Stir in enough demerara sugar to sweeten. Lightly grease a pan. Heat the pan, pour in about 2 tablespoons batter, swirl it around and cook for about 1 minute. Flip the crepe over with a spatula and cook the other side. Keep the crepe hot while cooking the remaining crepes.
Fill the crepes with the apple and black currant mixture and fold or roll them up. Serve with a dollop of creme fraiche, if using, and sprinkle with nuts or sesame seeds, if you like.