Combine the chickpeas and liquid with the water and bicarbonate of soda in a small pan. Boil for approximately 30 minutes.
Combine the yoghurt, tahini, garlic and lemon juice in a bowl and salt to taste. The tahini is the only thing you need to thicken the sauce.
Over a bowl to catch the liquid, drain the cooked chickpeas and set aside the cooking liquid. Using a fork, mash half