Whisk the eggs and sugar until they are very thick and the whisk leaves a trail when lifted. Gently fold in the flour. Line the base and sides of a tin with a base of 35 x 25 cm (14 x 10 inches). Pour in the mixture.
Bake in a preheated oven, 180°C (350°F), Gas Mark 4, for 12–15 minutes until the sponge is golden and springs back when pressed. Leave to cool in the tin.
Cut 12 x 7.5 cm (3 inch) circles from the sponge with a biscuit cutter. Cut 6 strips of plastic (a new plastic folder is ideal) measuring 27 x 7.5 cm (11 x 3 inches). Wrap a strip tightly around each of the sponge circles and secure with tape. Put on a baking sheet.
Sprinkle gelatine over the measured water in a bowl and leave to soak for 5 minutes. Stand the bowl in a pan of simmering water and leave until the gelatine has dissolved to a clear liquid. Whisk the egg whites until stiff peaks form, then gradually whisk in the sugar until thick and glossy (see
Divide the mixture between the plastic collars, then top with the remaining sponge circles. Chill the mousses for 4–5 hours until set. Remove the plastic, scatter each mousse with a few whole or halved berries, dust with sifted icing sugar and serve.