Place the thighs skin side down in a casserole dish. Add the wine, stock, tarragon and salt and pepper to taste. Bring to the boil and simmer 20 minutes. Remove the thighs to a heated plate, cover with foil and keep warm.
Simmer the sauce in the casserole until reduced by half. Add the cream and cook another 8 minutes. Adjust salt and pepper if necessary.
Mix together the butter and flour and add it to the sauce, stirring, but do not boil. Pour the sauce over the chicken and serve.