Combine the cream and caster sugar in a saucepan and bring to the boil, stirring. Reduce the heat and lightly boil for 3 minutes (this is important). Keep an eye on it and stir to keep it from boiling over. Remove from the heat and add the juice, stirring well. Leave to cool for 10 minutes, stir once more, decant into a jug and pour into the moulds. Cool and refrigerate for at least 4 hours until set.