Soak the gelatine in cold water for 5 minutes until softened then squeeze out any excess water. Put half the cranberry juice into a small saucepan with the sugar and heat gently until the sugar has just dissolved. Take off the heat and stir in the gelatine until dissolved.
Pour the remaining cranberry juice, vodka, orange liqueur and lime juice into a large jug. Add the cranberry and gelatine mixture through a strainer and stir. Divide between 4 martini glasses and chill overnight. Decorate with berries and serve.