Cut the jelly roll into slices or the cake into cubes or fingers, and fit them into the bottom and sides of a straight-sided trifle dish or other glass bowl. Drain the pear halves and pour about 1/2 cup of the syrup into a small bowl or measuring cup along with the sherry. Pour the mixture over the sponge cake or jelly roll, letting it soak up the syrup. Let it sit while you make the custard.
In a medium saucepan, whisk together the Bird’s custard powder, sugar and milk. Bring to a boil and cook for a few minutes, whisking constantly once it starts to bubble and thicken. If you like, spread (or drop in spoonfuls) the jam or preserves over the cake. Cut the pears lengthwise into thirds and lay them over the cake (or jam). Pour the custard overtop and let stand until it cools somewhat, then refrigerate until set. (You can do this part ahead and refrigerate overnight, or until you need it.)
Whip the cream with the sugar and vanilla until it holds softly stiff peaks. Spread over the custard and sprinkle with toasted almonds. Serves 8-10.