Melt half the butter and the syrup in a saucepan, add the biscuit crumbs and mix well. Tip into a greased 20 cm (8 inch) springform tin and press evenly over the base and up the sides almost to the tin's top. Chill.
Heat the remaining butter and the sugar in a nonstick frying pan until the butter has melted and the sugar dissolved. Add the condensed milk and cook over a medium heat, stirring continuously, for 4–5 minutes until the mixture thickens and begins to smell of caramel (do not have the heat too high or the condensed milk will burn).
Remove the pan from the heat and leave the mixture to cool for 1–2 minutes, then pour into the biscuit case. Allow to cool completely, then chill for at least 1 hour.
Whip the cream, just before serving, until soft peaks form. Peel and halve the bananas lengthways, then cut into slices and toss in the lemon juice. Fold two-thirds of the banana slices into the cream, then spoon over the toffee layer. Arrange the remaining bananas on top. Loosen the edge of the biscuit crust with a palette knife, then remove the tin and transfer the pie to a serving plate. Sprinkle with grated chocolate and serve cut into slices.