Cut the limes each into 8 wedges. Place in a jar with the lime juice and sprinkle the salt over them.
Mix well then cover the jar and leave in the sun for at least 2 weeks shaking the jar regularly.
Heat the oil in a saucepan over medium heat. Add the mustard seeds and stir fry for 30 seconds or until the seeds start to pop. Add the garlic, ginger, ground cumin, ground coriander and chilli powder.
Stir fry for about 30 seconds.
Add the limes, water, sugar and vinegar and bring to the boil.
Reduce heat to low and simmer, stirring occasionally, until the mixture is thick.
Spoon into sterilised glass jars.
When cool, store in the refrigerator for at least 1 week before eating.