Set aside a handful (about 50g) of the pitted cherries. Place the remainder in a large pan set over a medium heat with the cream and 50g icing sugar. Bring up to the boil, then gently simmer for 5 mins until the cherries start to soften. Carefully whizz in a blender, then pass through a sieve.
Pour a little of the hot cherry cream over the egg yolks, whisk well, then pour back into the pan. Cook for about 5 mins more or until the mixture has thickened a little and will coat the back of a wooden spoon. Pour through a sieve into a bowl, cover the surface with cling film, then leave to cool completely.
Whisk the egg whites until stiff peaks form, then whisk in the remaining 50g sugar, 1 tbsp at a time. Keep whisking until the mixture is stiff and resembles shaving foam. Stir a third of the mixture into the cooled cream, mix until it is well incorporated, then gently fold through the remaining mixture in 2 goes.
Line a 900g loaf tin with cling film. Pour the mixture into the tin, then cover with cling film and place in the freezer for about 3 hrs until just starting to freeze. Stir the whole cherries into the semi-frozen mixture, then return to the freezer for at least 8 hrs or preferably overnight. Serve cut into slices with some berries, or sandwiched between wafers.