Preheat the oven to 180°C.
Mix all of the dry ingredients together.
Whisk together the egg, natural yoghurt and oil. Fold the dry ingredients through the egg mix and add the blueberries.
Spoon the mixture into muffin cups until two thirds full, and set aside.
In a seperate mixing bowl, mix together the plain flour, oat flakes and demerara sugar. Rub in the unsalted butter until you obtain a lumpy crumb texture.
Generously sprinkle each muffin with the crumble mix. Bake for 20-25 minutes.
Check whether they are cooked enough by inserting skewer into the middle of the muffin. (The skewer should be clean when you pull it out).
Leave on top of a wire rack to cool.
«You can use frozen blueberries when fresh ones are out of season (or too pricey), just mix them into the batter straight from the freezer.»