Having mislaid my recipe book, MGS helpfully googled and found me the following recipe:
400g caster sugar
50g lemongrass, 3-4 sticks, cut in half lengthways
Make a syrup with the water and 325g of the caster sugar by bringing them to the boil in a saucepan and boiling for 5 minutes. Take the pan off the heat, add the lemongrass and let it infuse for about half an hour.
Strain the syrup into a measuring jug, keeping the saucepan with the lemongrass to one side.
Recorded here for posterity.