Put the mango cubes, sugar and lemon juice into a medium bowl and leave to macerate for 2 hours. The sugar will mostly dissolve and the flavours develop.
Empty the sugary mango mixture into the bowl of a food processor and blitz to a silky, smooth consistency. Add the fresh orange juice and passion fruit pulp and stir to combine.
Pour the sorbet into an ice-cream machine* and churn until the blade stops. Transfer to an airtight, freezer proof container, cover and put in the freezer for at least 4 hours, but preferably overnight until firm. Remove from the freezer and allow the sorbet to soften for a few minutes before scooping.
* Freezing ice cream without a machine: Pour the mixture into a shallow, wide, freezer proof container and freeze for 1 hour, until the mixture freezes, forming a border around the edge. Break up the ice crystals with a fork or whisk and return to the freezer. Repeat this whisking every 30 minutes or so, until evenly set and too thick to fork. It should take 2–3 hours.