Beat the egg yolks and 175g/6oz of the caster sugar in a heatproof bowl until pale and creamy. Add the vanilla pod, slit down one side, and cook over a pan of simmering water for 5-6 minutes, until the mixture thickens slightly and loses its eggy flavour. The best way to test if the mixture is ready is to taste it. Remove from the heat, take out the vanilla pod and allow to cool. When almost cool, add the mascarpone cheese and cream alternately, a little at a time, mixing thoroughly after each addition.
Place the blueberries in a saucepan with the water, remaining sugar and the lemon juice. Heat gently 1-2 minutes, until the sugar has dissolved, then leave to cool.
Cut the cake into slices about 2.5cm/1in thick and arrange half in the bottom of a large serving dish. Pour over half the sherry, and spoon over half the poached blueberries and half the mascarpone custard. Repeat all the layers, including the sherry, but reserve a little of the blueberry mixture for the top.
Decorate the top layer of mascarpone cream with the remaining poached blueberries, fresh blueberries and almonds. Alternatively serve the trifle in individual bowls or glasses.