Soak the gelatine leaves in cold water
In a small pan, stir together the caster sugar and milk over low heat until sugar is dissolved. Bring to a boil, then remove from heat. Stir in the soaked gelatine leaves. Let cool for 10 minutes. Beat in the yogurt and vanilla extract. Pour the mixture into 6 small molds, then chill for 3 hours
To make the sauce, puree the strawberries with lemon juice. Strain to remove the seeds.
To serve, dip the molds in warm water for 10 seconds, dry the base, then turn out the panna cotta onto a plate.
Spoon the strawberry sauce around each pudding, decorate with raspberries and blueberries.