Arrange the cake in the bottom of a trifle dish. Drizzle over the sloe gin or cassis and scatter with the blueberries (reserve a handful for the topping).
To make the jelly, put the gelatine in a bowl of cold water and soften for 5 minutes. Meanwhile, heat the blueberry juice in a pan. Remove the gelatine from the water, squeeze out any excess water and add to the hot juice. Stir until dissolved, then remove the pan from the heat. When it’s cool, pour the liquid over the fresh blueberries and chill for 3 hours, or until the jelly has set.
To make the custard, mix the custard powder with 700ml (about 1 pt) boiling water. Stir until dissolved, add the vanilla seeds and 50ml (2fl oz) cream. Cool a little, and then pour it over the jelly. Chill for at least 30 minutes, until set.
Meanwhile, put a third each of the raspberries and the strawberries in a blender with the sugar and blitz to a purée. Strain and discard the seeds. Halve the remaining strawberries and arrange them, along with the whole raspberries, over the custard. Spoon the purée over the top.
Whip the cream until soft peaks form and spoon over the trifle. Scatter with the reserved blueberries and then serve.