Add the chilli flakes and tomatoes to the onion mix and stir-fry for three minutes, just to soften the tomatoes. Drain the chickpeas reserving the liquid and make up to 300ml with water. Add the chickpeas, tamarind paste, 300ml chickpea water, cavolo nero, sea salt, sugar and tomato paste, bring to a boil, then lower the heat and simmer, stirring occasionally, for 20 minutes, until the liquid has almost completely reduced and is just clinging to the chickpeas. Taste and adjust the salt as needed.