Cut the ladyfingers into pieces so they fit a pretty glass bowl with a flat bottom. Drizzle with blackcurrant liqueur.
Place the currants and blueberries in a saucepan and slowly bring to the boil so they release their juice. Sweeten to taste. Cool slightly, then spread evenly on the ladyfingers.
For the custard heat the milk with the lemon zest and slowly bring to the boil. Remove from the hob.
In a heatproof bowl stir egg yolk and 175g sugar till smooth. Strain the milk and teaspoon by teaspoon, add the hot milk to the egg mixture. You need to start this slowly or the eggs will curdle. Once the egg mixture is about a cupful and hot, very slowly pour in the remaining milk while beating vigorously with a whisk.
Pour everything back into the saucepan and slowly bring to simmer over moderate heat, while beating with the whisk, till the custard thickens.
Remove from the hob and stir in the lemon juice. Let cool and whisk every now and then.
Slowly pour the custard over the berries, making sure they don't start floating. Let cool, then cover with cling foil and chill for several hours till serving.