Cut each lime into at least 16 pieces, more if they are big . Place in a large bowl. Sprinkle with the salt. Cover and set aside in a cool dry place, stirring occasionally, for 2 days.
Heat the oil in a medium saucepan over medium heat. Add the mustard seeds and cook for 30 seconds or until the seeds start to pop. Add the garlic, ginger, cumin, coriander and chilli powder. Cook, stirring, for 30 seconds or until aromatic.
Stir in the lime mixture, water, sugar and vinegar and bring to the boil. Reduce heat to low and simmer, stirring occasionally, for 15 to 30 minutes, until the mixture is thick.
Spoon into hot sterilised glass jars.