Line a deep 20 x 25cm baking tin with a sheet of baking parchment so it goes along the bottom and up the sides to make it easy to lift the trifle out. If you’re using a loose-bottomed tin, tightly wrap the base in foil. Cut the cakes horizontally and use to fit the base of the tin, drizzle with 3 tbsp of the sherry and set aside.
Soak the gelatine in cold water to soften and heat the custard gently in a saucepan or in the microwave until hot. Drain and squeeze the gelatine, then stir it into the custard until completely dissolved. Leave to cool slightly, then spread it evenly over the sponge. Put in the fridge for 30 mins to set.
Make up 1 pack of jelly following pack instructions, then leave to cool for 30 mins. Pour half over the custard, scatter over the berries, pour over the rest of the jelly and put in the fridge for 1 hr. Make up the remaining jelly, leave to cool, then pour over and put back back the fridge to set for at least 1 hr more. Can be made up to two days ahead and kept in the fridge.
Before serving, whisk the cream with the sugar and the remaining sherry until it holds its shape. Tip the cream into a piping bag fitted with a star nozzle. Carefully lift the trifle out of the tin and cut into 12 squares. Lift onto plates and pipe a big splodge of cream onto each square, and scatter with almonds and sprinkles to serve.