Trim all excess fat from roast. Place potatoes and onion in bottom of crockpot. Make a smooth paste of flour, mustard, chili sauce, worcestershire sauce, vinegar and sugar. Spread over top of roast (cut roast in half, if necessary, to fit easily). Place roast in crockpot on top of potatoes and onions. Cover and cook on LOW setting for 10 to dozen hours (on HIGH setting for 5 to 6 hours).
About 3 litre .