I don't like custard or jelly, so I'm making this by request. Even so, it sounds quite good.
Standard recipes list components without explaining how moisture interacts with structure over time. Sponge cake absorbs liquid rapidly. Adding sherry requires a measured hand to moisten the crumb without dissolving the cake into mush. Pour the fortified wine in a fine drizzle rather than a direct splash. This ensures even distribution across every square.
Visual cues dictate success when chilling gelatin and custard layers. Prepare the gelatin and monitor it closely in the refrigeration unit. It requires precisely 45 minutes to reach an intermediate, syrupy consistency before pouring over the cake. If the mixture is too liquid, it sinks straight to the bottom of the dish instead of binding the fruit and sponge. If fully set, it will not bond the architectural tiers together. Watch for a surface wobble that holds a gentle indentation when pressed with a clean finger. Custard presents its own visual test. Look for a glossy sheen that clings to the back of a spoon without running freely. Chill the assembled dish for at least four hours to allow the starches and proteins to firm into clean slices.