Soak the gelatine leaves in cold water in a shallow dish.
Put the artificial sweetener and milk in a small pan and stir over a low heat until the sugar has completely dissolved. Bring up to the boil, then take off the heat. Take the gelatine out of the water and stir into the hot milk.
Leave to cool for 10 minutes. Beat in the yogurt and vanilla extract and pour into 6 small moulds.
Chill until set (about 3 hours).
To make the sauce, puree the strawberries with the lemon juice and artificial sweetener. Sieve to remove pips. Mix the raspberries and blueberries in a separate large, pretty serving bowl.
To serve, dunk each mould into warm water for about 10 seconds, swiftly dry the base, then turn the panna cotta out on an individual plate.
Spoon a little of the strawberry sauce and some chocolate shavings around each pudding and serve.
Place the bowl of fruit in the middle of the tables and let people help themselves.