Remove the blade bone of the lamb leg and, using a sharp knife, make incisions all over the meat.
To make the marinade, put the papaya paste, crushed cinnamon, and ground spices in a large shallow dish and mix well.
Add the meat and rub the mixture evenly all over to coat. Cover and allow to marinate in the refrigerator for about 2 hours.
To make the whisked yoghurt paste, put the yoghurt, pastes, the chopped garlic, turmeric, cinnamon, lemon juice, oil and butter in another large dish. Season with salt and mix well.
Remove the meat from the first marinade, wiping off any excess moisture from the first marinade, and add the meat to the second marinade.
Rub the paste evenly all over the meat, then cover and allow to marinate in the refrigerator for a further 2 hours.
Preheat the oven to 190°C / 375°F / Gas Mark 5. Place the lamb leg with any remaining marinade into a large roasting dish or tin (pan) and roast the leg in the oven for about 1 hour, or until it is nearly cooked. Remove and cut the leg into 3-4 pieces along the bone.
Prepare a tandoor or charcoal grill for moderate heat, or alternatively preheat the grill (broiler) to medium.
Thread the lamb pieces onto several metal skewers and roast in a moderately hot tandoor or under the hot grill for 7-8 minutes. Remove from the heat and baste with melted butter, then roast for a further 3-4 minutes. Slice the meat and serve hot.