Using a sharp knife, make slits across one surface of the lamb and insert the garlic slivers at regular intervals.
Whisk together the oil, lemon juice and garlic together with the remaining marinade ingredients. Pour the marinade over the lamb and rub into the sufrace of the meat. Using 2-3 pieces of string, tie the rosemary branches to the meat. Cover with clingfilm and refrigerate for at least 2 hours.
When you are ready to cook (see above), remove the lamb from the fridge and bring to room temperature. Mix the ingredients for the rub in a small bowl. Sprinkle it on all sides of the meat and pat or rub it in to make sure it adheres to the meat. Cook on the kettle barbecue with the lid closed for 90-120 minutes, depending on desired degree of doneness.
When the meat is done, remove from the barbecue, cover lightly with foil and allow to rest for ten minutes before carving. As I said, we served ours in pita breads with tzatziki, but you could just as easily serve the lamb as part of a traditional Sunday roast.