Put the wine, celery, leek, carrot, stock, parsley and tarragon in a large frying pan over a medium–high heat and bring to the boil. Add the chicken breasts, reduce the heat to low and simmer, covered, for 25 minutes. Remove the chicken from the pan, cover with foil and keep warm.
Increase the heat to medium and boil the poaching liquid, uncovered, for 5 minutes or until reduced slightly.
Remove 1/3 cup (80ml) of the mixture to a small bowl. Add the flour and whisk until smooth. Stir this into the rest of the mixture in the pan and cook, stirring, for about 3 minutes or until the sauce boils and thickens slightly.
Remove from the heat and swirl in the margarine. Add pepper to taste. Transfer the chicken to warmed serving plates and spoon over the sauce.