Puree the peeled and seeded litchis in a food processor. Strain the puree through a sieve into the top of a double boiler or into a large steel bowl. Sprinkle the gelatine over the melon puree. When the gelatine has softened set the mixture over a pot or simmering water and stir until the gelatine has completely dissolved and the mixture registers 180 ºC on a candy thermometer. Stir in the lemon juice. Cover and refrigerate for several hours until set.