Put the chestnuts into a saucepan with the stock and bring to the boil, then reduce the heat, partially cover and simmer for 20-25 minutes until the chestnuts are just tender. Drain them, reserving the stock, and leave them to cool.
Remove any sinews from the turkey and pheasant. Check the pheasant for lead shot then cut it into long strips about 5 mm (¼ in) wide. Put 200 g (7 oz) of the neatest strips into a bowl, then add the prunes, 3 tablespoons of the brandy and half the allspice and season to taste. Stir then set aside.
Trim any membrane and white tissue from the livers. In a food processor or with a hand-held mixer, finely mince the remaining pheasant, the turkey and the livers. Mix well together, then mince once more and put the mixture into a large bowl.
Roughly chop a quarter of the chestnuts and set them aside. Mince the remaining chestnuts and stir them into the minced meats with the remaining brandy and allspice, a little salt and plenty of pepper.
Put a kettle on to boil. Trim the leeks to 30 cm (12 in) long, then cut them in half lengthways, discard the fine centre leaves and rinse well under cold running water. Lay them in a roasting tin or frying pan big enough to hold them in a single layer, cover with boiling water and leave for 1-2 minutes until just pliable. Refresh in cold water, separate the leaves and drain on kitchen paper.
Heat the oven to 180°C (350°F, gas mark 4). Lightly oil a nonstick loaf tin, measuring about 22 × 11 × 6 cm (8½ × 4¼ × 2½ in). Working widthways, line the tin with overlapping leeks, allowing the excess to hang over the sides.
Strain the brandy from the pheasant and prunes into the bowl of minced meat and stir it in. Add the egg, olive oil, pistachio nuts and 2 tablespoons of the reserved stock from the chestnuts and mix well. Gently stir in the chopped chestnuts – do not overmix or they will break up.
Spoon one-third of the mixture into the loaf tin and spread it out evenly. Arrange half the drained prunes, end to end, in a line along the centre, then lay half the pheasant strips on either side of the prunes.
Cover the pheasant and prunes with another third of the terrine mixture, then arrange the remaining prunes and pheasant strips on top as before. Spread the rest of the mixture evenly over the top.
Put a kettle of water on to boil. Cover the terrine lengthways with a layer of overlapping leek strips, trimming them to fit exactly, then bring the overhanging leek strips over the top.
Cover the tin tightly with foil and stand it in a small roasting tin. Pour in boiling water to halfway up the side of the loaf tin, then bake for 1 hour - 1 hour 15 minutes until a skewer inserted into the centre feels hot on the back of your hand.
Stand the loaf tin on a large plate, place a small board on top of the foil, then place some heavy weights, or cans of beans, on top. Allow it to cool, then refrigerate for 2-3 days to allow the flavours to mature.
Take the terrine out of the refrigerator 1 hour before serving. Remove the foil and drain off any juices. Loosen it by gently running a palette knife between the leeks and the tin, then turn the terrine out, cut into slices and serve.
Variation: the terrine can be made with duck and pheasant, goose and duck, all pheasant or all duck.