Preheat oven to 180 C / Gas 4. Spray a 20x30cm baking tin with cooking oil spray.
Arrange the peaches in the prepared tin. Scatter the butter over the peaches, then sprinkle the dark brown soft sugar evenly on top. Set aside.
Mix flour, caster sugar, baking powder and salt in a bowl. Stir the oil into the beaten eggs, then stir the egg mixture into the dry ingredients. Spoon the batter over the peaches, then sprinkle cinnamon on top.
Bake in preheated oven until the topping is lightly browned, about 45 minutes.