Add all the ingredients to a preserving pan, cover and simmer gently for 45 minutes, stirring from time to time.
Remove the lid and cook for a further 15 minutes until the limes are soft, stirring more frequently towards the end of cooking as the relish thickens.
Ladle into warm, dry jars, pressing down well and filling the jars to the tops. Disperse any air pockets with a skewer or small knife and cover with screw-top lids. Label and leave to mature in a cool, dark place for at least 3 weeks.