1) Preheat the oven to 180C/Gas 4.
2) Spray a 23-cm pie plate with cooking spray. In a food processor, process digestive biscuits until finely ground. Add butter and 1 tablespoon of water, and process until the crumb clumps together. Press crumb mixture into bottom of pie plate and about 1-cm up the sides. Bake in the oven for 10 minutes, then let cool.
3) In the meantime, make the filling. Put the gelatine in a small bowl; add 3 tablespoons of boiling water and stir until gelatin is dissolved. In a medium saucepan, whisk together sugar and the flour. In a medium bowl lightly beat the milk and eggs together. Add the egg and milk mixture to the saucepan and whisk so the flour and sugar dissolve. Cook over a medium heat, stirring constantly, for 10 minutes, until mixture comes to a boil and has thickened. Stir in the vanilla essence and gelatine. Set aside to cool slightly.
4) Arrange the sliced bananas on the digestive biscuit crust and pour the pudding on top. Place in the refrigerator until the pudding has set, about 3 hours.
5) Whip the double cream with an electric beater. When it is about halfway done, add 1/2 teaspoon of sugar, then continue whipping until fully whipped. Put the whipped cream in a plastic bag, concentrating it in 1 corner of the bag. Snip that corner off the bag and squeeze the whipped cream out of the bag in a decorative pattern around the pie.