1. Halve the limes lengthways, then slice thinly, reserving any pips. Tie the pips and lime leaves in a muslin bag and place the bag in a large pan with the sliced fruit.
2. Add the water to the pan and bring to the boil. Cover and simmer gently for 1 and a half-2 hours, or until the rind is very soft. Remove the muslin bag, leave to cool, then squeeze it over the pan to release any juice and pectin.
3. Add the sugar to the pan and stir over a low heat until the sugar has dissolved. Bring to the boil, then boil rapidly for 15 minutes, stirring occasionaly until setting point is reached (105C/220F).
4. Remove the pan from the heat and skim of any scum. Leave to cool for 4 minuste, stir, then pour into warmed sterilised jars. Seal, then label when cold. Store in a cool dark place.