Use a large baking tray; grease lightly. Mix pastry ingredients in a bowl; knead to make a dough. Knead into a smooth ball, wrap in cling wrap and chill for 30 minutes.
Preheat oven to 180°C. Roll out pastry on a floured surface into a circle 35 cm in diameter. Place on baking tray; prick several times with a fork. Bake for 15 minutes. Remove to a platter while still warm.
Peel 3 oranges, removing all the white pith. Hold oranges over a bowl to catch any juice; segment the fruit by slicing neatly between the membranes. Beat egg yolks and sugar until very pale and creamy. Arrange two-thirds of the orange segments over base.
Soak the gelatine in cold water. Bring 2 tbsp orange juice to the boil and dissolve gelatine in it. Stir into the remaining juice and add to egg mixture, stirring all the time. Chill until mixture begins to set.
Meanwhile, whip cream until stiff. When the mixture has set a little, fold in the cream. Place a piece of baking paper 5 cm high around the pastry base to support the filling while it sets. Spread the filling onto the tart and refrigerate until set. Remove paper collar. Decorate the tart with the remaining orange segments.