The posset should be made a day in advance to set properly.
First, cover the gelatine with cold water in a bowl and soak until soft.Blend the blackberries into a puree. Pour it into a saucepan with the cream, sugar and lemon juice. Bring to the boil and let it bubble for 3 minutes. Remove from the het and sieve into a bowl.
Remove the gelatine from the water and squeeze out excess water. Add the gelatine to the warm blackberry mixture and stir well until it has dissolved.
Now pour the mixture into the glasses, put into the fridge and allow to set.
Decorate with the blackberries or other fruit like raspberries, blueberries and mint.