To make the base, crush the biscuits until like fine crumbs then tip into a bowl. Stir in the melted butter. Press the mixture into the base and 4cm (1½in) up the sides of the tin. Chill the base while you make the filling.
To make the filling, put the tin in a pan of boiling water and boil for 1 1/2 - 2 hours, (do not pierce the tin, don't panic it won't explode)Don't let the pan boil dry either. Allow to cool slightly, not too much and open carefully with a tin opener. Spread the filling over the biscuit base, and then leave to chill for about 1 hour, until firm.
To serve, remove the pie from the tin and place on a serving plate. Slice the bananas; fold half of them into the softly whipped cream and spoon over the toffee base. Decorate with the remaining bananas and dust liberally with the cocoa.