To make the crust:
Line the bottom of a 9 inch springform pan with parchment paper.
Blend the graham crackers in a food processor until they are finely ground.
Pour the melted butter over the crumbs and process to blend well. The crumbs should stick together when pressed.
Press the crumb mixture over the bottom and 1 1/2 inches up the sides of the springform pan. Refrigerate.
To make the toffee sauce:
Place a medium heavy saucepan over medium heat.
Combine the sugar and 3 tablespoons of water in a medium heavy saucepan.
Stir over medium heat until the sugar dissolves.
Increase the heat and boil without stirring until the color is deep amber, occasionally swirling the pan and brushing down the sides with a pastry brush dipped into water, about 5 minutes.
Stir in the condensed milk and butter.
Continue stirring for 5 minutes or until the sauce thickens slightly.
Remove the toffee sauce from the heat and spread 1 cup of the sauce over the prepared crust and refrigerate for about 1 hour or until the caramel is semi-firm.
Keep the remaining caramel sauce at room temperature.
To fill the pie:
Using an electric mixer, beat the cream in a large bowl until thick and very soft billowy peaks form.
Very thinly slice 3 of the bananas into discs.
Fold the sliced bananas into the softly whipped cream and spoon into the prepared pie crust.
Slice the remaining bananas and arrange them decoratively over the pie.
Rewarm the remaining toffee sauce gently over low heat.
Drizzle some of the sauce decoratively over the pie.
If the sauce has thickened too much to drizzle, stir a few tablespoons of milk into the sauce to create a thinner consistency.
Cut the pie into wedges and transfer to plates.
Drizzle each pie wedge with more sauce and serve.