Place chicken in a large saucepan, cover with water, add a third of a tablespoon of dried tarragon (or a table- spoon of fresh), a pinch of salt and bring to the boil. Reduce heat immediately to a gentle simmer, skim off foam and poach gently until the chicken is just cooked. about 1 hour.
Twenty minutes before the end of cooking time add 7 ounces diced carrots and 5 minutes later 2 large leeks, sliced, and 7 ounces peas. When chicken is cooked, drain off stock into another saucepan and boil hard to reduce for 10 minutes.
Cool stock fast by sitting the saucepan in a sinkful of cold water, then transfer to the freezer to set the fat so it can be skimmed off Skin the chicken, remove the meat from the bones and chop. Put the chicken and vegetables in a souffle or oven dish.
Melt 3-1/2 tablespoons butter or margarine in a saucepan and add 3 tablespoons plain flour to make a roux. Add 2 cups chicken stock and 6-1/2 ounces milk and cook gently, stirring, until the sauce thickens. Add more stock if too thick. Pour over the chicken mixture.
Meanwhile cook and mash 9 ounces white-fleshed sweet potato and allow to cool. Sift together 4 ounces plain flour, 1 teaspoon salt and 1 teaspoon baking powder. Rub in softened butter. Add a little water if necessary to make a soft dough. (Or process the ingredients in a food processor, adding the water if necessary.)
Tip onto a floured surface and with floury hands pat it out to fit inside the top of the dish containing the chicken and vegetables. Place on top of the chicken mixture. Bake in oven for about 45 minutes or until the top is golden.Servings: 6 to 8