In a small bowl, mix together black pepper, paprika, chilli powder, celery salt, cayenne pepper, garlic powder and mustard powder. Rub mixture over the surface of the meat. Place roast in a slow cooker and add 1/2 cup (125ml) water.
Cover and cook on Low for 8 to 10 hours. When meat is tender and well done transfer roast to a serving platter. Skim fat from juices and strain; serve with meat as is or make gravy.