Remove any sinew left on the meat then score a criss-cross pattern into the outside of the leg about 2cm wide.
In a bowl, combine all of the ingredients then rub all over the lamb. Allow to marinate for 3 hours. Bring the lamb up to room temperature prior to cooking.
Preheat the barbecue to a low/medium heat (160 C) and grease the grill. Lay the leg of lamb on the grill and cover.
Barbecue in increments of 20 minutes, turning the lamb regularly to ensure an even cook. Check the lamb after 30 to 45 minutes, at this point the lamb will be fairly rare so continue cooking in 20 minute increments until cooked to taste.
Remove the lamb from the barbecue and cover in aluminium foil for 10 to 15 minutes to rest before serving.