Stir flour, sugar, and salt together in a medium-sized bowl. Add butter; cut with pastry cutter until it resembles fine meal or is pea-sized.
Add 4 tablespoons ice water and cider vinegar. Stir with a wooden spoon until moist clumps form, adding more ice water by teaspoonful if dough is dry.
Gather dough together and turn out onto work surface. Form into a ball and flatten into a disk. Wrap disk in plastic and refrigerate at least 1 hour.
Using a little flour to dust the surface, thinly roll out the pastry and place in a loose-bottomed tart tin.
Leave the excess overhanging and prick the base with a fork. Chill for 30 minutes until firm.
Heat oven to 400 degrees F.
Put the tart tin on a baking sheet, then line the pastry with foil and fill with baking beans or rice. Bake for 15 minutes or until the pastry is firm and dry, then remove the beans and foil. Bake for 20 minutes more or until the pastry is golden brown and biscuity. Leave to cool.
Pour the milk into a pan and bring to a simmer.
In a large mixing bowl, whisk together the cornstarch, sugar, orange zest, juice and marmalade. Pour the warm milk into the bowl, whisking constantly.
Pour the mixture into a clean pan and cook over low heat, stirring all the time, until shimmering and thickened. The custard might look a bit lumpy at first, but keep stirring and it will come together.
Take off the heat and beat in the butter. Constantly whisking, add 1/2 cup of the warm custard to egg yolks, then add egg mixture back to the pan of custard. Be sure to whisk constantly until fully incorporated.
Beat the egg whites to stiff peaks in a large bowl. Add the sugar in four additions, beating back to stiff peaks after each, to make a thick meringue. Whisk in the cream of tarter.
Trim the edges of the pastry case with a small serrated knife.
Warm the orange filling until it bubbles, stirring occasionally. Spoon the filling into the case and smooth the top.
Carefully spoon the meringue on top of the hot filling, starting at the edge and working towards the middle to prevent the meringue from sinking. Gently swirl the meringue down to meet the pastry all around the edge of the tart.
Bake for 15 to 20 minutes or until the meringue is pale golden brown.
Cool for at least 1 hour, then remove from the tin and serve.
*I couldn't find golden caster sugar so substituted white caster sugar, aka C & H Baker's sugar. Or you can process regular granulated sugar in the food processor for 2 minutes until it is finely ground.