To make jelly, peel and roughly chop the flesh of 1 mango. Spoon into a blender or processor with the juice of 1 lime and 2 tablespoons of rum. Blend until smooth. Pour the mango nectar into a medium saucepan and sprinkle over the gelatine. Place over medium heat, whisk gently for 5 minutes until hot. Add the mango puree mixture and whisk over medium heat for 1 minute. Remove from heat and cool for 10 minutes then strain into a jug (you should have about 3 1/2 cups).
Peel and slice 2 mangoes and arrange over the base of a 10-cup capacity trifle bowl. Pour over 2 cups of the jelly. Pour remaining jelly into a Chinese container. Cover and refrigerate both for 4 hours until set.
To make the custard, whisk egg yolks, sugar and flours together in a bowl. Add milk, slowly, whisking constantly until smooth. Pour mixture into a non-stick saucepan. Cook, stirring constantly over medium heat until custard just comes to the boil. Pour into a bowl, press a piece of plastic wrap onto the surface. Refrigerate for 4 hours.
Fold half the mascarpone into the custard and spoon over the jelly. Combine caster sugar and 2 tablespoons of boiling water in a heatproof jug, stir until sugar has dissolved. Cool for 5 minutes. Stir in the remaining 1/4 cup of rum and the juice from the remaining 2 limes. Pour into a shallow dish. Dip the biscuits, one at a time into the rum mixture then press randomly into the custard. Cover and refrigerate overnight.
Just before serving, fold the whipped cream and remaining mascarpone together. Spoon over the custard. Peel and chop fruit from the remaining 2 mangoes. Dice the jelly in the Chinese container. Spoon the mango and jelly over the trifle. Drizzle with passionfruit. Serve.