Bring the water to the boil in a saucepan. Add the sugar and stir constantly until dissolved. Add the vinegar and simmer until the syrup has thickened and coats the back of a spoon.
Prick the outside of 6 finger limes with a small pin or the tip of a sharp knife. This will get the oils working.
Take the syrup off the heat, add the finger limes, stir well and set aside to cool. The syrup should now have the consistency of thin honey and be infused with the aroma of the finger lime skins.
When cold, strain the syrup into a sterilised bottle. From here you can either seal ready to drink later, or serve straight away.
To serve put the following into each glass: 1/3 syrup, 2/3 soda water, Finger lime slices, Finger lime pearls (scrape out of the open fruit), Mint leaves & Ice