When buying your leg of lamb, there is no need to ask the butcher to de-bone it. In fact, if you cook it on the bone, this will improve the flavour.
Do choose locally produced lamb if at all possible, which has not already come halfway round the world before ending up in your kirchen. Trust your butcher!
The 7-hour cooking time is not absolutely fixed, so if you raise the temperature, you can reduce the cooking time: 5 hours at 150°C (300°F), for example.
You can accompany the lamb with just about anything you choose: vegetables, potatoes, pasta, rice, etc. Simply serve the cooking juices in a jug or drain them off from the bag 10 minutes before the end of cooking and reduce them over low heat.