Merry cherry Christmas tart

Name: Merry cherry Christmas tart
Prep time: 2 h
Cook time: 40 min
Total time: 2 h 40 min
Cuisine: European, British
Ingredients: 455g (3 1/2 cups) cherries, pitted; 100g (1/2 cup) caster sugar; 2 tbsp cornflour; 2 tbsp port; 1 egg white; 1-2 tbsp demerara sugar; Icing sugar, to serve; Custard, to serve (optional); 225g (1 1/2 cups) plain flour; 150g unsalted butter, chilled, chopped; 1 egg yolk; 2-3 tbsp pouring cream, chilled