Loin of lamb is a prime cut, usually roasted (or served as chops). It can be cooked on the bone or boned, stuffed and rolled.
For lamb on the bone, allow 20 minutes a pound (450 g) plus 20 minutes at 220°C (425°F) mark 7. Boned and rolled allow 25 minutes a pound (450 g) plus 25 minutes at 220°C (425°F) mark 7. Put in a roasting pan with the thickest part of the fat uppermost. If very lean, add dripping and baste 2 - 3 times during cooking.