Put the sugar in a saucepan with the measurement water, elderflower cordial, lavender and rosemary and bring to the boil. Boil for 8–10 minutes until the liquid has reduced by a third and become syrupy but without taking on any colour. Remove from the heat and leave to cool completely.
Strain the cooled syrup through a sieve into a freezer-proof container and freeze for about 4–5 hours until firm. Remove from the freezer and cut into chunks, then use a stick blender to beat until smooth. Return to the freezer and freeze again for about 3–4 hours or overnight until firm. The sorbet is now ready to serve, but allow to soften for 15 minutes before serving.