To make the pastry, combine the butter and lard and divide into four portions. Sift the flour and salt into a bowl, rub in a single portion of the fat and mix with the water and lemon juice. Knead the dough on a lightly floured surface for 5 minutes, or until smooth. Roll out to about 40 x 10 cm. Take a second portion of the fat and dot evenly over the top two-thirds of the dough. Fold the bottom third up and over the centre to cover the fat, then bring the top third down to cover the bottom third. Seal the open ends with the rolling pin. Give the dough a quarter turn, roll out and fold as above, twice more, using up the remaining portions of fat. Wrap in plastic wrap and chill for 30 minutes. The pastry can be prepared to this point the day before it is needed.
To make the filling, heat the butter in a small saucepan, stir in the flour and cook for 1 minute. Gradually stir in the fish stock and cider or white wine. Bring to the boil, stirring continuously. Remove from the heat, season with pepper, leave until cool, then mix in the prawns and crab.
Roll out the dough until it is a little less than 5 mm thick. Using a 10 cm plain round cutter, stamp out rounds and set aside. Lay the trimmings neatly on top of each other, reroll and cut more rounds to make 16 in all. Lay 8 rounds on a baking sheet, spoon some filling onto the centre of each, brush the edges with cold water and cover with the remaining rounds. Press the edges together lightly with the back of a knife and chill for 30 minutes.
Heat the oven to 220°C. Glaze the pies by brushing with the beaten egg, make two small slits in the top of each pie and bake for 25 minutes, or until golden brown.