Preheat the oven to 350°F /180°C (160°C fan).
Spray a 12-cup muffin pan with nonstick spray or use cupcake liners.
Dry ingredients: In a large bowl, whisk together the flour, sugar, baking powder, cinnamon, allspice (mixed spice), salt and nutmeg (if using) until well combined.
Wet ingredients: In a separate small bowl or measuring jug, whisk together the buttermilk (or eggs or buttermilk substitute), milk and oil.
Combine dry and wet ingredients: Make a well in the center of the dry ingredients and pour in the wet ingredients.
Add in the fruit mix: Add in the orange zest, raisins and sultanas (or cranberries)
Mix gently: Fold everything together gently just until combined. Spoon the batter evenly into liners, filling them until they are three quarters full.
Once you have scooped the batter into the muffin liners, use the back of a spoon or a small spatula to very gently and lightly level the surface of the batter. This will ensure that your muffins have a smooth, even top when they are baked.