Preheat the oven to 180 C / Gas 4. Lightly grease and flour a loaf tin.
Sift the flour and salt into a bowl. Rub in the margarine with fingertips until it resembles breadcrumbs; mix in sugar.
Combine the egg, yoghurt and banana in a bowl; mix well. Fold banana mixture into flour mixture until just combined; pour into prepared loaf tin.
Bake in the preheated oven until a skewer inserted into the centre of the loaf comes out clean, about 60 minutes. Leave loaf to cool overnight before serving.