Place the water in a small bowl and sprinkle the gelatin powder over the top. Set aside for a couple minutes to "bloom" (swell up) while you prepare the mango and measure out other ingredients. It will look odd as it swells, but don't worry.
Cut the mango into dice, removing skin and the stone. You can do this as you prefer, but I generally cut cheeks off either side of the stone, score in a cross pattern then pop open to give cubes (see the piece open up in ingredients photo above) that you can then easily cut off. Try to remove any obviously stringy pieces as you go.
Once you have cut enough mango, place in a blender and blend until really smooth. If you have any doubt about it being smooth or if the mango was a little stringy, then also strain the mango with a fine strainer to remove any stringiness.
Place the coconut milk, mango puree, sugar and bloomed gelatin mixture in a small pan. Warm over a relatively low heat, stirring almost constantly, to mix everything together and dissolve both the sugar and gelatin. Take care that it doesn't get too hot - DO NOT BOIL. The gelatin will start out as being in clumps but will gradually melt into the mixture. Once you can't see any pieces of gelatin and the mixture is smooth, remove from the heat.
Divide the mixture between custard dishes, ramekins, glasses or moulds as you prefer. Gently tap the top if needed to pop any bubbles. Transfer the puddings to the fridge to set - they need at least two hours.
Serve chilled, either as they are or with a drizzle of coconut milk or evaporated milk, and a few cubes of fresh mango.