Preheat the barbecue for indirect cooking.
While the barbecue is preheating, remove the lamb from the fridge.
Using a sharp knife, cut 10 to 12 small slits into the flesh of the lamb. Push a piece of garlic and a small sprig of rosemary into each slit.
In a small bowl mix the lemon juice, olive oil, salt and pepper. Using a pastry brush, coat the lamb with the olive oil mixture.
Once the barbecue is preheated, turn the centre burner off and the outside burners to medium.
Place the leg of lamb on the centre of the grill. Close the lid and cook for 1 ¼ -1 ½ hours to cook to medium, depending on the thickness of the meat.
To estimate the cooking time, measure the meat through its thickest part and cook for 1 minute per millimetre for medium. For example, if the roast is 90 mm thick, cook for 90 minutes.