Combine all the ingredients together, except the oil and mustard seeds, in a saucepan. Bring to the boil, reduce the heat to moderately low and simmer for 5 hours, stirring occasionally, or until the mixture is thick.

Meanwhile, heat the oil in a small frying-pan. Add the mustard seeds and cover. Fry until they begin to pop, then tip the seeds and oil into the saucepan. Cook for a further 15 minutes.

Remove from the heat and spoon the chutney into clean, dry jam or preserving jars. Serve when cool, or store, covered with vinegar-resistant paper, in a cool place or in the refrigerator.