Line a 2 lb (1 kg) loaf tin with clingfilm or nonstick baking paper. Put the sugar in a saucepan, add 250 ml (8 fl oz) water and warm gently until the sugar is dissolved. Remove from the heat and stir in the lime juice and grated rind.
Meanwhile, process the mango flesh to make a smooth purée, reserving four thin slices for the decoration. Stir the purée into the lime syrup and pour the mixture into the loaf tin. Freeze for at least 4 hours or overnight until solid.
Remove the sorbet from the tin and blend or process with the egg whites. Return the mixture to the tin and return to the freezer until firm. Eat within 3 days because the flavour of fresh fruit sorbets deteriorates quickly and this one has raw egg in it. Before serving, decorate each portion of sorbet with a thin slice of mango and serve with a couple of wafer biscuits.