Prepare the leg of lamb by scoring the top side of the joint. Combine the oil, pepper, cumin, thyme, rosemary, tarragon and chilli in a bowl then rub generously all over the lamb. Allow to marinate for 3 hours. Just prior to cooking, rub the lamb with salt.
To make a baste, in a bowl, combine the vegetable oil, garlic, lemon juice honey and parsley.
Preheat the barbecue to a steady medium heat (about 160 C). Grease the barbecue grill then rub the lamb with salt and lay the lamb on the grill. Close the lid.
Barbecue the lamb, covered, for 1 hour. Baste and turn the leg every 15 to 20 minutes. Remove from the heat after an hour for rare to medium-rare. Continue to cook for a further 20 to 30 minutes for medium to medium-well.
Remove from the barbecue, cover with aluminium foil, and allow to rest for 10 to 15 minutes before carving and serving.