Sift the flour into a bowl, stir in the lemon rind and set aside.
Whisk the egg whites, salt and cream of tartar in a large clean bowl until stiff but moist-looking. Gradually whisk in the sugar, a tablespoonful at a time, until it has all been added. Whisk for a few minutes more until the meringue mixture is thick and glossy.
Gently fold in the flour mixture using a metal spoon and a swirling figure-of-eight action. Pour into a 20 or 23 cm (8 or 9 inch) nonstick angel cake tin. Bake in a preheated oven, 190°C (375°F), Gas Mark 5, for 25–30 minutes until well risen, the cake is golden and springs back when gently pressed with a fingertip.
Invert the tin on to a wire rack and leave to cool. As it cools the cake will fall out of the tin. When cold, mix the lemon curd and soured cream together and spread over the top of the cake. Sprinkle with crystallized rose petals or flowers, if using.
To crystallize flowers such as rose petals or viola, pansy or herb flowers, first make sure they are clean. Brush them with egg white then dust lightly with a little caster sugar. Leave to dry for at least 30 minutes before using to decorate the cake.