Pound the saffron and mix it with the boiling water. Set aside. Trim all the fat off the leg of lamb. Mix the yoghurt, salt, pepper, garlic and chillies with a quarter of the saffron. Smear this mixture and the butter all over the leg of lamb, then wrap it in cooking foil, to enclose all the juices. Bake in a moderately hot oven (200c, 400F, gas 6) for 1 hour.

Unwrap the foil and pour another quarter of the saffron over the meat. Continue cooking for a further 15 minutes, again wrapped in foil. Open the foil and bake for a final 20 minutes. Shortly before serving, pour the remaining saffron over the meat.