Preheat the oven to 200`c. Place the frozen berries in a sieve over a bowl and leave to defrost if using. Mix together the eggs, caster sugar, milk, melted butter and vanilla extract in a large bowl. Sift the flour, baking powder and salt over the top and briefly fold together, making sure that you don’t over beat (The less you mix everything together, the lighter the cooked muffins will be) Don’t worry if the mixture looks rather lumpy at this stage. Tip in the defrosted and strained berries, (or the fresh berries if using) and again fold in briefly. Divide the mixture between 14-16 muffin cases, making sure not to fill them more than three-quarters of the way up. Sprinkle the tops with the demerara sugar and oat flakes. Cook in the oven for about 20 minutes until risen and golden. Remove from the oven then cool in the tin for 5 minutes before transferring to a wire rack. Enjoy just warm, or cooled.