Squeeze the juice of the lemon. Put the sherry, sugar, spices, and lemon juice into a large mixing bowl. Stir until well-blended, then leave to soak for at least half an hour. Beat the cream until it is stiff. Fold in the chopped ginger. Chill thoroughly. Before serving divide into four serving bowls and top with crumbs of crystallized ginger and a shake of nutmeg.