Preheat the oven to 190°C/Fan 170°C/ Gas 5. Line the base of a 23 x 33cm baking tray with a strip of parchment paper, so some excess hangs over the longer side of the baking tray.
For the slab scone, mix the flour, the teaspoons baking powder, sugar, salt and lemon zest together in a large bowl, then rub the unsalted butter into the flour mixture until it resembles coarse breadcrumbs with a few larger pieces remaining.
Make a well in the middle and pour in the milk and three of the beaten eggs, stirring to form a soft but not sticky dough.
Tip the dough out onto a lightly floured work surface, press or roll into a 23 x 33cm rectangle and transfer to the prepared tin.
Beat the remaining egg with a splash of milk to form an egg wash, and brush over the top of the scone, then sprinkle liberally with the demerara sugar. Bake for 20–25 minutes, or until golden brown. Set aside to cool completely in the tin.
Meanwhile, prepare the topping. Place the strawberries into a large bowl and sprinkle over the sugar and vanilla extract, stirring together briefly. Leave to macerate for 30-35 minutes, or until the sugar has dissolved.
Once cool, remove the scone from the tin and place it onto a wire rack. Spread the clotted cream all over the scone and then top with the macerated strawberries, drizzling with the syrup that is left in the bottom of the bowl.
Cut into portions and serve. This scone slab is best served on the day it's made, soon after assembling.