In a plastic container or bag, combine lamb, garlic, lemon and rosemary. Marinate in the fridge for 3 hours or ideally overnight.
Preheat barbecue to medium. Remove lamb rumps, blot excess marinade with paper towel and place on a hot barbecue for 10 minutes each side. Remove and loosely wrap in tinfoil to rest for 10 minutes. Unwrap the lamb, slice against the grain and serve with cous cous salad and tzatziki.
Bring a large pot of water to the boil, add couscous and cook until firm to the bite (about 3-5 mins). Drain well and toss in a large bowl with rest of the ingredients while still warm. Season with cracked pepper and set aside.