Unwrap and rinse corned beef.
Arrange medium coals around edge of grill.
Place beef on heavy-duty foil drip pan on grill.
Close hood; grill for 1 1/2 hours.
Scrub potatoes but do not peel.
Cut each in 3 or 4 lengthwise slices.
Set aside 3 tablespoons soup mix.
Blend together remaining soup mix and butter.
Spread mixture over potato slices.
Wrap each in a square of heavy-duty foil.
Place at edges of grill.
Grill, hood down, along with meat for 45 to 60 minutes more; turn potatoes once.
Meanwhile, in saucepan mix sugar, vinegar, 3 tablespoons mustard, and salt.
Bring to boiling; stir till sugar dissolves.
Brush over meat during last few minutes of grilling.
Just before serving, mix sour cream, milk, reserved soup mix, and 2 tablespoons mustard.
Heat through, stirring occasionally.
Do not boil.
Arrange meat and potatoes on serving platter.