In a large mixing bowl, combine 8oz of plain flour and a pinch of salt. Cut in 4oz of butter, using your fingers or a pastry cutter, until the mixture resembles coarse crumbs. Gradually add cold water, mixing just until the dough holds together. Shape the dough into a disk, then wrap it in plastic film. Chill in the refrigerator for 30 minutes to relax the gluten.

Preheat your oven to 190°C (375°F). Roll out the chilled pastry on a floured surface to fit your tart tin. Carefully transfer the dough into the tin, trimming any excess. Line the pastry with parchment paper, fill it with baking beans, and bake for 15 minutes. Remove the paper and beans, then bake for an additional 10-15 minutes until the crust is lightly golden. Set aside to cool.

In a medium saucepan, melt 4oz of butter over low heat, then add 10oz of demerara sugar. Stir gently until fully dissolved, about 5 minutes. Gradually whisk in 6floz of milk and 3oz of plain flour, combining until smooth. Continue to cook over medium heat, stirring constantly until the mixture thickens, which should take around 5-7 minutes.

Once the filling is thick and creamy, carefully pour it into the cooled pastry shell. Use a spatula to smooth the top, ensuring an even distribution. Let it sit at room temperature for about 10 minutes, then cover with plastic wrap and place in the refrigerator for at least 2 hours.

To serve, you can garnish each slice of the tart with a dollop of whipped cream or a sprinkle of flaky sea salt. Cut into wedges and serve chilled.