Put the stock, chicken, parsley, chervil and two-thirds of the leeks into a large saucepan. Bring to the boil, reduce the heat, skim any scum from the surface, then cover and simmer for 1 hour, or until the chicken is tender.
Transfer the cooked chicken to a plate and set aside. Skim the fat from the surface of the soup. Reserve the legs of the chicken – they are not needed for the soup. Remove the skin from the breast, then take the meat off the carcass and cut it into thin strips. Return the meat to the pan and heat gently.
Whisk the egg yolks in a bowl with the cream, then gradually stir in three ladlefuls of the soup. Stir the egg and cream mixture into the soup, season to taste with the salt and black pepper or ground allspice and heat gently, stirring until the soup thickens slightly. DO NOT BOIL as it will curdle.
Cook the remaining leek in boiling water for 1 minute and drain well. Pour the soup into a heated tureen or bowls and sprinkle with the leek and the chopped parsley and chervil. Serve with Melba toast or your favourite bread.