In a medium bowl, mix together the digestive biscuit crumbs, 4 tablespoons sugar and melted margarine. Press into the bottom of a greased 20x30cm baking dish.
In another bowl, mix together the cream cheese and 150g sugar until smooth. Blend in the eggs one at a time, then stir in the vanilla and mashed banana. Pour into the prepared base.
Bake for 30 minutes in the preheated oven, or until the centre is set. Cool, and refrigerate for at least 3 hours.
Combine evaporated milk and toffee in a saucepan over medium heat. Cook stirring frequently until toffee is melted and the mixture is smooth. Garnish the cheesecake with banana slices, and drizzle toffee sauce.