Wash the fruit. Pare the zest from 2 of the lemons and 1 lime taking care not to include too much white pith. Place in a saucepan with the caster sugar and 500ml water.
Heat gently stirring until the sugar dissolves, then remove from the heat.
Squeeze the juice from the lemons and limes (you should have about 250ml) and stir into the pan.
Allow to infuse for at least 30 minutes. Remove the lemon zest with a draining spoon and discard. Pour the lemon and lime mixture into a large serving jug. Add a few sprigs of mint and top up with 750ml of cold water.
Chill well. Serve over ice with extra lemon and lime slices if desired.
If your fruit does not yield very much juice then reduce the additional water you add to the syrup accordingly. If you have a lot more juice dilute to taste.