Combine the yoghurt with all the ingredients up to and including lemon juice in a bowl and mix well. Dissolve the gelatine in the boiling water, mix together with 2 tbsp of the yoghurt mixture and immediately stir into the remaining yoghurt. Chill until the mixture is slightly firm at the edges (about 15 mins.), stir until smooth. Using a rubber spatula, carefully fold the cream into the yoghurt mixture and divide into 4 glasses. Cover the mousse and leave to set in the fridge for approx. 2 hrs.
Mix the melon with the icing sugar and lemon balm, cover and leave to steep in the fridge for about an hour.