Put the cream in a heavy-based pan with the sugar, lemon and lime rind. Place on a low heat, stirring until the sugar has dissolves and the cream is just under scalding point.
Remove the pan from the heat and stir in the lemon and lime juice. Leave to cool, then strain through a sieve into a jug. Pour into small coffee cups or ramekins or individual small glass dishes (each one should take about 120ml of liquid) and leave to set in the fridge for at least 6 hours, or overnight is best.
Serve the lemon and lime possets straight from the fridge and place on saucers or small plates, then decorate with extra lime rind and the mint leaves.