Sift the flour and salt into a large mixing bowl. Use your fingertips to rub the Trex into the flour until the mixture looks like fine crumbs. Stir in the caster sugar. Add 4-5 tbsp chilled water and mix it in with a knife until the pastry clings together. Form the dough into a ball, then wrap and chill for 10 minutes.
Put the cherries into a large bowl and add the caster sugar and cornflour, tossing to mix. Tip into the pie dish and sprinkle 4 tbsp cold water on top. Dampen the edges of the pastry, lift the lid on top and press the edges together to seal. Trim with a sharp knife. Re-roll the trimmings and use them to make pastry leaves.
Brush the surface of the pie with the glaze, decorate with pastry leaves and brush them with the glaze. Sprinkle with caster sugar and bake for 30-35 minutes, until golden brown. Serve with fresh cream, ice cream or custard.