Cut the mango and avocado flesh into 2 cm (¾ inch) pieces. Discard the outer layer of leaves and cut the stems off the lettuces, leaving the hearts. Separate the leaves and add them to the mango and avocado with the prawns.
Make the dressing by whisking together the lime juice, sugar and oil with the chilli. Add the dressing to the salad, toss carefully to mix and serve immediately.