Heat an empty, heavy-based saucepan until you can feel a strong heat rising. Sprinkle in the sugar. It should start to caramelise. Swirl the pan as the sugar dissolves and turns dark golden, but do not stir.
When most of the sugar has caramelised, carefully pour in 500ml/18fl oz just-boiled water, and stir with a long handled spoon. It will spit, so take care. The sugar will become firm, but keep heating and stirring and it will dissolve again. Add the lemon zest and vanilla. Simmer for 5 mins.
Score the peaches in half and twist to separate. Remove the stones. Slip the peaches into the syrup in a single layer.
Return the liquor to a gentle simmer and turn down the heat until it barely simmers. Cook for 3-5 mins until the flesh feels tender when pierced. Remove the pan from the heat, mix in the lemon juice and allow to cool. Mix in the blueberries and serve at room temperature or chilled.