Measure 5 tablespoons of the purée into a small saucepan and add the lemon juice and sugar. Place over a low heat and leave the sugar to completely dissolve so that you have a mango syrup. Pour this into a bowl with the remaining mango purée and mix it through thoroughly.In a separate bowl, whip the cream with the rose water and kerwa water (if using), until it's almost doubled in volume and it forms stiff peaks. Fold the mango purée through the whipped cream, being as gentle as possible, until no streaks remain.Pour the mixture into a freezer-proof container (30 x 20cm and at least 4cm deep), cover with a lid and freeze for at least 4 hours to set. Serve as it is or with rose-flavoured syrup and a sprinkling of chopped pistachios.