Depends on your weight of lamb but for 3.5 pounds, I would say 180C Gas4 for 3 to 4 hours....it's lovely with a green bean, pomegranate and pine nut salad.
Dont think you will find a boneless half shoulder of lamb weighing 3.5 pounds
Just put it in a slow cooker for 5 hours It will be perfect . Loses less weight than roasting as well