Make the sorbet syrup. In a small bowl, stir the sugar and stabiliser powder together thoroughly. Put the water and the glucose or runny honey in a saucepan. Heat gently to bring just to the boil.
Pour the contents of the bowl into the saucepan in a steady stream, stirring all the while with a whisk. Bring back to just boiling, then remove from the heat. Leave the syrup to cool until tepid before using. (It will keep for up to a week in the fridge.)
To make the gelato, stone the peaches and cut into chunks. Blend the peach flesh finely with the sorbet syrup, then pass it through a sieve into a bowl. (Discard the contents of the sieve.) Churn the mixture in your ice-cream machine until fully firm.
Before serving, put the peach gelato in the freezer for half an hour or so to firm up. If it has been stored in the freezer longer and is too firm, allow it to soften in the fridge until scoopable.