Hull and chop any large strawberries and put them into a bowl with the raspberries and blueberries, then sprinkle over the icing sugar and gently mash with a fork. Set aside to macerate.
Lay the Madeleines in the bottom of a large trifle bowl, then spread them out to fill the gaps as much as possible. Drizzle over the Marsala wine, if using. Once the berries look juicy, spoon half of them, with their juice, over the cake and leave to soak in.
Whip together the custard and mascarpone until smooth, then spoon half over the berries, spreading to the edges. Spoon over the rest of the berries, then top with the remaining custard.
Whip the cream to soft peaks and spoon two-thirds over the trifle and level. Use the extra berries to create a Union Jack pattern, leaving a gap between each colour. Using a narrow piping nozzle, pipe the remaining cream between the berries to create the white parts of the flag.