Heat the oven to 180C/160C fan/gas 4 and line a muffin tin with 9 cases. Mix all the dry ingredients until combined, then gently stir in the milk, egg and oil.
Gently fold in the leek and Cheddar. Spoon the mixture evenly into the muffin cases and place in the oven. Bake for about 25 mins, then check - they may need a little longer. These taste great served with either spinach soup or with a dab of butter.