Two days before you cook the lamb outside on the BBQ, make a 'butterfly cut' along the bone, or ask the butcher to do it!
Place the butterfly-cut leg of lamb in a close fitting container, pour the lemon juice over the top and sprinkle with a little salt. Cover and chill for 24 hours.
The next day, toast (dry-fry) the spices in a small pan over low heat for 2 - 3 minutes, stirring until you smell the heavy aroma. Grind as finely as possible in a food processor or spice mill, and place in a bowl.
Add the yogurt and mix thoroughly to make the marinade.
Remove the meat from the container and wipe any juices from the base. Spoon in half the marinade, then lay the lamb down onto this. Cover with the remaining marinade, ensuring that all the meat has marinade on it.
Chill for a further 24 hours.
Return to room temperature at least 2 hours before cooking.
Light the BBQ an hour before cooking.
Brush the marinade off the lamb, and discard it.
Sprinkle the outside of the meat with a little seasoning (salt, pepper, new spices), and lay on the grill, seasoned side down, about 8 - 10 inches from the coals.
Leave untouched to cook for 15 minutes.
Season the upper surface of the lamb before turning. Cook this side for 15 minutes.
Remove and set aside for 10 minutes, to seal in all the juices.
Carve across the grain in thick slices.
Serve with a warm chickpea salad or a couscous salad, and slices of cucumber, tomato and salad greens.