Dry rub: add the mustard powder, garlic powder, onion powder, paprika, cayenne pepper, salt and pepper to a glass jug and use a mini whisk to stir till consistently combined then set aside.
Ribs: working one rack at a time place the ribs bone side up on a clean, dry cutting board. Start at one corner, grab hold of the membrane (silver skin) and pull to remove this and then discard it. Use a knife to slice the rack in half around the middle to ensure it will fit into the slow cooker insert.
Rub into ribs: sprinkle some of the dry rub over the back side of each set of ribs. Use your fingers to massage in well. Then flip to the otherside and repeat.
Liquids: pour the worcestershire sauce and apple cider vinegar into the base of the slow cooker.
Slow cooker: place the rub covered ribs into the slow cooker (stacking the ribs is ok) and pop the lid on. Cook the ribs on high for 4 hours or low for 8 hours.
Preheat the oven: preheat the oven to 200°C 392° F6 with the grill setting.
Sauce and grill: once the cooking time for the ribs is up, use a pair of tongs and remove them from the slow cooker and place the ribs onto a silicon baking mat on a wire rack. Use a basting brush to cover the ribs in the barbecue sauce and then place the ribs onto top shelf of the oven and grill for 10 to 15 minutes.
To serve: once the ribs have finished grilling, use an oven mitt to remove the tray and place it onto a heatproof surface. Let them sit for 5 minutes then transfer them to a serving plate with a small bowl of sauce.