Place the chopped peaches (fresh or tinned) in a bowl and beat with blender until pureed. Add sugar if needed. Place some of the peach puree (100 g) in a small saucepan, heat for a little and leave to stand for half an hour.
Heat the puree up again to 40°C. Add a steady stream of sugar while continually stirring. Bring the puree to a boil and remove from heat.
Squeeze the water out of the gelatine and mix well. Add the remaining peach puree (150 g) and once again mix well until even. Pour the peach puree into a silicone mould. Chill in the refrigerator until firm.