Put the granulated sugar, water, lemon rind and juice into a saucepan and heat gently until the sugar is dissolved, then boil for 3 minutes. Remove from the heat and stir in the rum, then leave to cool.
Place the strawberries in a food processor or blender with the cooled syrup and blend until completely smooth. Transfer to a plastic container and freeze. Beat the mixture after 1 hour, then beat again every 30 minutes until you have a soft sorbet texture. Alternatively, churn in an ice cream maker until slushy Freeze until required.
Cut the mangoes down each side of the stone and then cut each piece in half. Combine the brown sugar and spice powder and dust lightly over the mango portions. Cook on a medium barbecue for 2—3 minutes on each side until charred. Serve with the strawberry ice.