Trim excess fat from meat. If necessary, cut meat to fit into a 3-1/2 or 4 quart slow cooker. Set aside.
In medium bowl, combine water, Worcestershire sauce, vinegar, bouillon granules, mustard, chili powder, cayenne pepper and garlic.
Reserve 1/2 cup mixture for barbecue sauce. Cover and refrigerate.
Place meat in slow cooker. Add remaining Worcestershire sauce mixture.
Cover and cook on low heat setting for 10 to 12 hours or on high heat setting for 5 to 6 hours.
Before serving, in small saucepan, combine the 1/2 cup Worcestershire sauce mixture, ketchup, sugar substitute and butter. Heat thoroughly over medium heat, stirring occasionally. Serve sauce with meat.