Place the defrosted mixed berries and their juices in a medium mixing bowl. Add the orange pieces, the nectarine segments, the sweetener and the mixed spice and gently stir.
Place 3 sponge fingers in the bottom of the serving bowl. Place a large serving spoonful of the fruit mixture on top of the sponge fingers (this will provide support for the sponge fingers that will stand on their ends around the inside of the bowl).
Place 10 upright sponge fingers at equal intervals around the inside of the serving bowl.
Place half of the fruit into the serving bowl, gently pushing it into the spaces between the sponge fingers and levelling the fruit mixture.
Place the remaining 7 sponge fingers on top of the fruit. Spread the remaining fruit over the sponge fingers and level the top.
Place the egg yolks, vanilla, cornflour and sweetener in a heatproof measuring jug and stir to form a smooth mixture.
Place the milk in a medium saucepan and place over a medium heat, until just steaming hot, taking care not to let it burn or boil over.
Pour the hot milk into the mixture in the jug, stir, then immediately pour back into the saucepan and place over a medium heat.
Stir the custard mixture constantly with a wooden spoon or balloon whisk, for about 5 minutes until gently bubbling and thickened.
Remove from the heat and leave to cool slightly. Stir occasionally whilst cooling to stop a skin forming on top of the custard.
Pour the warm custard over the fruit and sponge then leave to cool.
Once cooled, cover with cling film and place in the fridge for about 40 minutes or until the custard has set.
Place the mascarpone, icing sugar and milk in a small bowl and stir with a wooden spoon until smooth and the consistency of lightly whipped cream.
Spread the topping over the top of the trifle, swirling with a small palette knife.
Decorate the top of the trifle with fresh raspberries, sprigs of mint and toasted flaked almonds.
Cover with cling film and chill for 3 - 4 hours, or overnight.
Serve alone or with a squirt of aerosol cream if wished.