Soak the chickpeas in water overnight. Drain and boil in plenty of water until really soft. This can take anything from 45 minutes to over 2 hours, depending on the age of the chickpeas. Drain well. Grind the cumin and coriander seeds finely. Peel the garlic and chop roughly. Put the tomatoes in a bowl, cover with boiling water and leave for 2 minutes, then peel and chop roughly. Put the tamarind paste into a measuring jug, add the hot water and stir to mix. Put the oil and butter in a wide, heavy, heatproof casserole over a medium heat. Then stir in the turmeric and the ground cumin and coriander. Next add the garlic and chopped tomatoes, the tamarind water, mustard seeds, cayenne pepper and a sprinkling of salt. Add the cooked chickpeas, cover the dish and cook gently on top of the stove, stirring once or twice, for about 15 minutes. Meanwhile, wash the spinach and add the whole leaves to the casserole. Cover again and continue cooking for about 5 minutes, until the spinach is soft. Remove the lid and, if the mixture is slightly liquid, bubble it for a few minutes, stirring in the open dish until it is fairly thick and mushy. Lower the heat and make 4 depressions in the chickpea and spinach mixture. Break the eggs carefully into these depressions. Cover again and cook for 6-8 minutes, until the eggs are just set - they should still be runny in the centre. Sprinkle with roughly chopped coriander leaves and serve immediately.