Pre heat the oven to 180 degrees C / 356 F, I use a fan assisted, electric oven, please adjust according to your own oven.Line a 12 hole muffin tray with muffin cases.
Melt the butter over a low heat, once melted set aside to cool.
Sift all the dry ingredients, except the sugar, into a large bowl.
Break the eggs into another bowl and beat. Add the cooled melted butter, milk, vanilla extract, lemon zest and sugar and beat again until the sugar is more or less dissolved.
Make a well in the dry ingredients and pour in the wet ingredients. Using a large metal spoon fold in the flour. Keep going until you have a wet, thick batter. Fold in the grated apple until it is fully incorporated into the batter. Then carefully mix in the blackberries.
Spoon 2 heaped tablespoons of the batter into the muffin cases. Place the tray on the middle shelf and bake for 30 - 35 minutes. The muffins are done when you pierce them with a metal skewer and it comes out clean.Leave the muffins to cool for a few minutes in the tray then transfer to a cooling rack. Serve warm or cold.