Sprinkle the gelatine on 4 tablespoons cold water, then dissolve, stirring, over low heat. Cool. Mix the mango purée, sugar, lime juice and gelatine. Whip the cream until stiff, then fold lightly but thoroughly into the mango mixture. Pour into a mould rinsed out with cold water. Refrigerate until set. Unmould and serve plain, or garnished with slices of mango.