Start this recipe by making the jam. Place the defrosted raspberries and caster sugar into a small pan with a little lemon zest and 3 tbsp. water. Bring to the boil and cook for 5 minutes. Allow to cool down completely. Meanwhile, whip the 2 tubs double cream with 2 tbsp. icing sugar and vanilla to soft peaks. Put half into the fridge to decorate. Next, mix the mascarpone in a large bowl with the remaining 2 tbsp. icing sugar and lemon zest. Carefully fold in half of the whipped cream. Now you can put the trifle together. Place half of the cake into the base of a large trifle bowl. Spoon over half of the raspberry jam, followed by half of the mascarpone cream. Spoon over ⅓ of the berries, followed by half of the pouring custard. Repeat the process again using the remaining cake, mascarpone cream, jam, pouring custard and another ⅓ of the berries. To finish off, spread over half of the reserved whipped cream and pipe the remaining cream on top. Decorate with the remaining strawberries and blueberries and chill before serving.