Preheat the oven to 180 C / Gas 4. Butter 2 mini muffin tins.
In a bowl, stir together the oats, wholemeal flour, plain flour, baking powder, salt and cinnamon. Make a well in the centre.
In another bowl, beat together the eggs, milk, brown sugar and vegetable oil. Stir in the courgette and carrot, then the sultanas. Pour this mixture into the well of the dry mixture, and stir together until all ingredients are moist. Let the muffin mixture rest for 5 to 10 minutes at room temperature. Fill the mini muffin moulds about 2/3 full.
Bake in the preheated oven until the muffins are golden and a skewer inserted near the centre comes out clean, about 15 minutes. Wait 5 minutes before unmoulding the muffins to cool completely on a wire rack.
You can use a regular muffin tin, which will yield 12 to 15 muffins. Simply increase baking time as needed to 25 to 35 minutes, or till a skewer inserted in the centre comes out clean.