Prepare Your Tin and Dry Ingredients: Preheat your oven to 200°C (180°C fan/Gas Mark 6). Line a 12-hole muffin tin with paper cases. In a large mixing bowl, whisk together the plain flour, caster sugar, baking powder, bicarbonate of soda, and salt until they are thoroughly combined and no lumps of baking powder remain. The mixture should look uniform and pale.

Combine Wet Ingredients: In a separate medium bowl, lightly whisk the large egg, then add the whole milk, vegetable oil (or melted butter), and vanilla extract. Whisk until the mixture is smooth and pale yellow, making sure everything is well incorporated.

Gently Mix Wet and Dry: Pour the wet ingredients into the dry ingredients. Using a spatula or wooden spoon, mix until just combined. The batter will look thick and slightly lumpy, which is exactly what you want; avoid overmixing. You might still see a few streaks of flour, but that's perfectly fine.

Fold in Strawberries: Gently fold in the diced fresh strawberries. Distribute them evenly throughout the batter, taking care not to crush them too much. The batter will turn slightly pink in places where the strawberries release their juices, adding a lovely visual cue.

Prepare Cream Cheese Swirl: In a small bowl, whisk together the softened cream cheese, icing sugar, and 1 tablespoon of milk until completely smooth and creamy. It should be a pipeable consistency, holding soft peaks.

Fill Muffin Cases and Swirl: Divide the muffin batter evenly among the 12 prepared paper cases. Each case should be about two-thirds full. Dollop about a teaspoon of the cream cheese mixture onto the top of each muffin. Using a skewer or a knife, gently swirl the cream cheese into the batter, creating an appealing marbled pattern. Sprinkle with demerara sugar if desired for an extra crunch and sparkle.

Bake to Golden Perfection: Place the muffin tin into the preheated oven. Bake for 20-25 minutes, or until the muffins are golden brown on top, well-risen, and a skewer inserted into the centre comes out clean (avoiding the cream cheese swirl). You'll notice a delightful sweet and fruity aroma filling your kitchen as they bake, and the tops will feel springy to the touch.

Cool and Serve: Once baked, remove the muffins from the oven and let them cool in the tin for 5 minutes. Then, transfer them to a wire cooling rack to cool completely. They are particularly lovely warm, with the cream cheese still soft and gooey.