Peel, stone and roughly chop the mangos; set aside. Remove the cardamom seeds from the pods. Peel and finely chop the garlic, then trim and finely chop the chilli. Add the vinegar and sugar to a large pan over a medium heat, stirring until the sugar dissolves. Bring to the boil and reduce by a couple of centimetres. Gently toast the cumin, coriander and cardamom seeds until aromatic, then crush with the chilli powder using a pestle and mortar. Add to the vinegar pan, along with the chopped mango, nigella seeds and 2 teaspoons of sea salt. Finely grate in the ginger, add the garlic and bring to a boil. Reduce the heat and simmer for 45 minutes to 1 hour until it has a thick, syrupy consistency, adding the chopped chilli for the last 10 minutes. Divide among sterilised jars, seal and keep for up to 6 months.