Preheat the oven to 220C/425F/Gas 7. Grease and line a 33x23cm/13x9in Swiss roll tin with baking parchment.
For the Swiss roll, whisk the eggs and sugar together in a large bowl until the mixture is light and frothy and the whisk leaves a trail when lifted out. Sift the flour into the mixture and carefully fold it in. Turn the mixture into the prepared tin and give it a gentle shake so that the mixture finds its own level, making sure that it has spread evenly to the corners.
Bake for 10-12 minutes, or until the sponge is golden-brown and begins to shrink from the edges of the tin. While the cake is cooking, place a piece of baking parchment a little bigger than the size of the tin on the work surface and dust it with caster sugar.
Turn the cake out onto the sugared parchment and peel off the paper. Trim the edges of the sponge with a sharp knife to give straight clean edges. Make a score mark running down one of the long sides, about 2cm/1in in from the edge, being careful not to cut right through.
Leave to cool slightly, then spread with jam. Roll up the cake firmly from the scored end.
Cut the Swiss roll into 14 slices. Carefully arrange eight of the slices, closely together, around the sides of a 20cm/8in, straight sided, glass bowl, with the cut, rolled sides facing outwards.
For the filling, arrange the remaining six slices in the base of the dish. Crumble the ratafia biscuits over the top.
Combine 150ml/5fl oz of the pear juice drained from the tin with the sherry and sprinkle it over the sponge and crumbled biscuits. Level the surface by pressing down with a spoon and leave to soak while you make the custard.
For the custard, put the egg yolks, caster sugar, cornflour and vanilla extract into a large bowl and whisk until blended. Heat the milk and cream together in a pan until hot, but not boiling. Gradually whisk this into the egg yolk mixture a little at a time, then return the mixture to the pan. Stir over a high heat until the mixture just comes to the boil and the custard thickens. Take off the heat and cover with cling film in contact with the surface to prevent a skin forming.
Drain the pears and cut each half in three lengthways and arrange over the sponges. Pour over the custard, cover and leave until completely cold before transferring to the fridge to chill and set.
Beat the whipping cream in a bowl until a ribbon of cream falls from the whisk when the whisk is removed.Leaving three tablespoons aside for final decoration, gently spread the cream over the surface of the set custard and scatter with the almonds. Whisk the remaining cream further until soft peaks form when the whisk is removed and spoon 10 cream blobs around the trifle, one for each portion. Decorate with 10 cherries and chill until ready to serve.