Whisk the milk, custard powder and sugar in a saucepan. Cook on low flame till the mixture has thickened. Switch off and allow to cool.
Whip the cream till stiff. Fold in the cooled custard. Add the vanilla.
In a small glass bowl, crush the wafers and spread on the bottom. Keep a little for garnish.
Chop the bananans coarsely and toss the lemon juice in. Spread the bananas on top of the water.
Spread the prepared custard mixture on top. Refrigerate for a couple of hours or until settled.