Put the lemongrass into a saucepan with the sugar and 1/2 litre of water, stir until the sugar dissolves, then bring to the boil. Reduce the heat and simmer for 2 minutes. Cool, cover and chill overnight for the flavours to infuse. Strain the syrup and add the lime juice and half the lime zest.
Divide the fruit between 4 bowls, pour over some syrup and scatter the remaining lime zest. Any leftover syrup will hold in the fridge for up to three weeks. Serve with a bowl of natural yoghurt.