Fry the chicken in oil in a large frying pan until the skins are browned. Pour off the excess oil. Add the stock, 300 ml (½ pint) milk, 1 tsp Tarragon, Thyme and White Pepper.
Bring to the boil, cover and simmer for 45-60 minutes until the chicken is tender. (Alternatively, it can be cooked in the oven at 180°C, 350°F, Gas Mark 4). Remove the chicken and keep warm. Pour the liquid into a meauring jug and make up to 750 ml (1¾ pints) using the remaining milk.
Melt the butter and stir in the flour. Gradually blend in the milk mixture and remaining Tarragon. Bring to the boil, stirring and simmer for 1 minute. Add half the grapes. Pour over the chicken and garnish with the remaining grapes.