Put the rice in a sieve and rinse well in cold water. Transfer to a saucepan, pour in the coconut milk, then set over a medium heat and bring to the boil.
Reduce the heat to very low, stir the rice and cover with a lid. Simmer for 20 minutes, until the rice is sticky and cooked through. (You may need to give the rice a stir while it cooks to stop it from sticking to the base of the pan.)
Taste the rice, adding agave syrup until it is at your preferred level of sweetness.
Divide among four bowls. Thinly slice the mangoes, arrange on top of the coconut rice and serve.