Wash the limes. Cut them in half and juice them (reserve the juice). Cover the skins in cold water and refrigerate overnight. The next day drain the skins, cut into quarters and scrape out the flesh and membrane. Put this on to a 40cm (16in) square of muslin and tie into a bag with string. Slice the skins into fine shreds lengthways. Place a small plate in the fridge to chill.
Put the shredded limes into a preserving pan with the muslin bag, lime leaves, 2 litres (3½ pints) water and the juice, cover tightly and cook gently for an hour and a half or until the fruit is soft. Limes have tough skins, so make sure you are happy with the softness – once the sugar is added the skins will harden slightly. You don't want to lose too much water, so cook it gently.