Mix the butter and oil then stir in the rest of the marinade ingredients. Season with black pepper. Place the lamb in a shallow dish and spoon over the marinade. Rub it all over the meat. Cover; chill overnight. Remove from the fridge 1 hour before cooking.
Preheat the barbecue and, once hot, add the lamb, fat-side down. Cook on a high heat for 5 minutes until golden. Turn and cook for a further 5 minutes.
Reduce the heat and cook for 30-40 minutes, turning occasionally. Rest for 10-15 minutes. Slice thickly; serve with Greek yogurt and rice - or serve in wraps with a simple salad and pickled red onion - yum!