Squeeze the juice from 1½ limes and blend with the mango to give a smooth purée.
Cut the rest of the limes into quarters, and then cut each wedge in half again. Put 2 pieces of lime in a highball glass for each person and add 1 teaspoon of caster sugar and 5-6 mint leaves to each glass. Squish everything together with a muddler or the end of a rolling pin to release all the flavours from the lime and mint.
Divide the mango purée between the glasses and add 30ml white rum and a handful of crushed ice to each one, stirring well to mix everything together. Top up with soda water to serve and garnish with extra mint, if you like.
