Add the double cream and condensed milk to a large bowl, and whisk for several minutes until thick and frothy - you should be able to write your name with the mixture that drips from the whisk before it sinks in.
In a separate bowl, thoroughly mash the bananas. Add them to the ice cream mixture, and fold in gently.
Pour half of the ice cream mixture into a tub or loaf tin, and drizzle over half of the toffee sauce. Top with the remaining ice cream mixture, and remaining toffee sauce.
Place in the freezer overnight, or until completely frozen.