1. Slice 2cm slits into the lamb with a sharp knife. Combine marinade ingredients including 1/2 lemon juice in a small bowl and mix well. Rub over the lamb, rest at room temperature for 10 minutes.
2. Preheat the barbecue to 180ºc. Turn off burners underneath the grill plate, the remaining burners will conduct and circulate heat around the roast.
3. Place lamb on the grill plate, close lid and cook to preferred doneness, 25 minutes per 500 grams for medium (approximately 2 hrs for a 2kg leg).
4. check the internal temperature with a meat thermometer (rare 60ºc, medium rare 60-63ºc, medium 65-70ºc, medium well done 70ºc, well done 73ºc
5. Meanwhile, peel the cucumber and slice into ribbons with a carrot peeler. Mix through the rocket salad cherry tomatoes and feta. In a small bowl whisk remaining lemon juice, oil, salt and pepper, combine with salad.
6. When lamb is cooked transfer to a plate, cover loosely with foil and rest it in a warm place for 15 minutes. Carve the roast across the grain and serve with salad.