In a medium bowl, sift together the flour, 3 tablespoons sugar and salt. Rub in the margarine until the mixture resembles coarse crumbs. In a small bowl, whisk together the egg and cold water. Sprinkle over flour mixture, and work with hands to form pastry into a ball. Chill 30 minutes.
Preheat oven to 180 C / Gas 4. Roll out half of pastry to 3mm thickness. Place in a 23x33cm baking dish, covering bottom and halfway up sides. Bake for 20 minutes, or until golden brown.
In a large saucepan, mix the peaches, lemon juice and orange juice. Add 110g butter, and cook over medium-low heat until butter is melted. In a mixing bowl, stir together 400g sugar, nutmeg, cinnamon and cornflour; mix into peach mixture. Remove from heat, and pour into baked pastry case.
Roll remaining pastry to a thickness of 5mm. Cut into 1.5cm-wide strips. Weave strips into a lattice over peaches. Sprinkle with 1 tablespoon sugar, and drizzle with 15g melted butter.
Bake in preheated oven for 35 to 40 minutes, or until top pastry is golden brown.