Try this recipe from Marie Summerwood.
Serves 4.

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9 oz sugar
18 fl oz water
6 lavender heads
juice from one lemon

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Stir the sugar into 8 fl oz of the water, in a saucepan, until the sugar dissolves. Add the lavender heads. Bring to the boil, then add the juice of half the lemon, and allow to cool. Strain the liquid through a sieve, to remove the lavender heads, and add the remaining water. Taste to check the sweetness, and add more lemon juice if necessary. Put in a freezer-proof container and still-freeze.

To still-freeze, it's best to use a large shallow container at first. Pour the liquid in, place it in the coldest part of the freezer, then wait for at least an hour (it may take longer depending on the freezer, mine took about 4 hours to start to freeze) until the mixture is just starting to freeze, with crystals on it.

Working quickly, put the mixture in the liquidizer or food processor, give it a quick process, then put it back to keep freezing. Repeat the processing twice more after another couple of hours each time. This
breaks up the ice crystals to give a scoopable sorbet. Of course, if you have an ice-cream maker, you can use that.