Fish Veronique

1 lb white fish (such as cod, sole, or turbot)
1/4 tsp salt
1/8 tsp black pepper
1/4C dry white wine
1/4 C chicken stock or broth, skim fat from top
1 Tbsp lemon juice
1 Tbsp soft margarine
2 Tbsp flour
3/4C lowfat or skim milk
1/2 C seedless grapes
as needed nonstick cooking spray