Remove excess fat from lamb, rinse with water to clean and pat dry with paper towel.
Pierce broadest side of meat joint multiple times with sharp knife.
Place rosemary, lemon zest, garlic and olive oil into a bowl and mix thoroughly.
Spread mixture all over meat joint on all sides.
Sprinkle salt all over meat joint on all sides, then sprinkle broad side of meat joint with black pepper.
Place lamb in on top of wire rack inside roasting tin, broadest side face up, and place in preheated oven at 380.
Cook until medium well for 2 hours, or 1 1/2 hours for medium.
Allow to Lamb to rest for 8-10 mins before carving into slices.